Install pydot in travis and skip test if it isn't avail.

.travis.yml

@@ -20,7 +20,7 @@ before_install:
 install:
 # We support scipy 0.7.2, but it is not available on conda.
 # So we test with 0.11. Our internal buildbot have 0.7.2.
-  - conda create --yes -q -n py26 python=2.6 numpy=1.6 scipy=0.11 nose=1.1 pip
+  - conda create --yes -q -n py26 python=2.6 numpy=1.6 scipy=0.11 nose=1.1 pip pydot=1.0.28
   - source activate py26
   - pip install . --no-deps --use-mirrors
 

theano/tests/test_printing.py

@@ -46,6 +46,10 @@ def test_pydotprint_cond_highlight():
 
 
 def test_pydotprint_return_image():
+    # Skip test if pydot is not available.
+    if not theano.printing.pydot_imported:
+        raise SkipTest('pydot not available')
+
     x = tensor.dvector()
     ret = theano.printing.pydotprint(x * 2, return_image=True)
     assert isinstance(ret, str)