Travis CI: reduce hardcoding of Python 3 versions (#10705)

* Try testing on Python 3.7

* Try theano

* Update .travis.yml

* Don't pip install tensorflow

* Travis CI: reduce hardcoding of Python 3 versions

Python 2.7 needs __pil__ but _all other_ currently supported versions of Python need __Pillow__ instead.

.travis.yml

@@ -50,7 +50,7 @@ install:
   # install PIL for preprocessing tests
   - if [[ "$TRAVIS_PYTHON_VERSION" == "2.7" ]]; then
       conda install pil;
-    elif [[ "$TRAVIS_PYTHON_VERSION" == "3.6" ]]; then
+    else
       conda install Pillow;
     fi