It seems there is no version `1.5.3` in conda channel. Version prior to `1.5.4` is `1.5.1`:

```
$ conda search sphinx
Fetching package metadata .............
sphinx                       1.1.3                    py26_1  defaults        
                             1.1.3                    py27_1  defaults        
                             1.1.3                    py27_2  defaults        
                             1.1.3                    py33_2  defaults        
                             1.1.3                    py26_2  defaults        
                             1.1.3                    py27_3  defaults        
                             1.1.3                    py33_3  defaults        
                             1.1.3                    py26_3  defaults        
                             1.1.3                    py33_4  defaults        
                             1.1.3                    py26_4  defaults        
                             1.1.3                    py27_4  defaults        
                             1.2                      py26_0  defaults        
                             1.2                      py27_0  defaults        
                             1.2                      py33_0  defaults        
                             1.2.1                    py33_0  defaults        
                             1.2.1                    py27_0  defaults        
                             1.2.1                    py26_0  defaults        
                             1.2.2                    py34_0  defaults        
                             1.2.2                    py26_0  defaults        
                             1.2.2                    py27_0  defaults        
                             1.2.2                    py33_0  defaults        
                             1.2.2                    py34_1  defaults        
                             1.2.2                    py33_1  defaults        
                             1.2.2                    py27_1  defaults        
                             1.2.2                    py26_1  defaults        
                             1.2.2                    py34_2  defaults        
                             1.2.2                    py26_2  defaults        
                             1.2.2                    py33_2  defaults        
                             1.2.2                    py27_2  defaults        
                             1.2.3                    py26_0  defaults        
                             1.2.3                    py34_0  defaults        
                             1.2.3                    py33_0  defaults        
                             1.2.3                    py27_0  defaults        
                             1.3.1                    py35_0  defaults        
                             1.3.1                    py26_0  defaults        
                             1.3.1                    py27_0  defaults        
                             1.3.1                    py33_0  defaults        
                             1.3.1                    py34_0  defaults        
                             1.3.5                    py34_0  defaults        
                             1.3.5                    py27_0  defaults        
                             1.3.5                    py35_0  defaults        
                             1.4.1                    py34_0  defaults        
                             1.4.1                    py27_0  defaults        
                             1.4.1                    py35_0  defaults        
                             1.4.6                    py34_0  defaults        
                             1.4.6                    py27_0  defaults        
                             1.4.6                    py35_0  defaults        
                             1.4.8                    py35_0  defaults        
                             1.4.8                    py34_0  defaults        
                             1.4.8                    py27_0  defaults        
                             1.5                      py35_0  defaults        
                             1.5                      py34_0  defaults        
                             1.5                      py27_0  defaults        
                             1.5.1                    py35_0  defaults        
                             1.5.1                    py36_0  defaults        
                             1.5.1                    py34_0  defaults        
                          *  1.5.1                    py27_0  defaults        
                             1.5.4                    py35_0  defaults        
                             1.5.4                    py27_0  defaults        
                             1.5.4                    py36_0  defaults        
```