Update .travis.yml

Author: cxhernandez

.travis.yml

@@ -18,7 +18,7 @@ install:
 
 script:
     - conda build --quiet devtools/conda-recipe
-    - devtools/travis-ci/build_docs.sh
+    - if [ $CONDA_PY == 3.6 ]; then devtools/travis-ci/build_docs.sh; fi
 
 after_success:
   - cp $HOME/miniconda3/conda-bld/test-tmp_dir/.coverage .