Use pip3 to install for python3 on ubuntu 14.04

doc/install_ubuntu.txt

@@ -60,7 +60,7 @@ On 14.04, this will install Python 2 by default. If you want to use Python 3:
 .. code-block:: bash
 
     sudo apt-get install python3-numpy python3-scipy python3-dev python3-pip python3-nose g++ libopenblas-dev git
-    sudo pip install Theano
+    sudo pip3 install Theano
 
 For Ubuntu 11.04: