BLD: Run travis tests on python 3.7.

Author: Scott Sanderson

Diff for: .travis.yml

@@ -5,6 +5,7 @@ python:
   - "3.4"
   - "3.5"
   - "3.6"
+  - "3.7"
 
 before_script:
   - pip install tox
@@ -14,3 +15,4 @@ script:
   - if [[ $TRAVIS_PYTHON_VERSION = '3.4' ]]; then tox -e py34; fi
   - if [[ $TRAVIS_PYTHON_VERSION = '3.5' ]]; then tox -e py35; fi
   - if [[ $TRAVIS_PYTHON_VERSION = '3.6' ]]; then tox -e py36; fi
+  - if [[ $TRAVIS_PYTHON_VERSION = '3.7' ]]; then tox -e py37; fi