Add python3.6 build config to .travis.yml

We only really need one build environment (python3.6); the rest are
redundant, since Travis CI seems to be able to test all python versions
from 2.7 through 3.6 with this one build environment.

.travis.yml

@@ -4,7 +4,8 @@ python:
   - "3.3"
   - "3.4"
   - "3.5"
+  - "3.6"
 install:
   - "pip install tox"  # for running tests only
   - "python setup.py -q install"
-script: tox
+script: export PY_TEST_VERSION=py$(echo ${TRAVIS_PYTHON_VERSION} | sed 's/\.//') && tox -e style,$PY_TEST_VERSION