Add most recent Python versions in Travis CI

Add more recent Python versions including
development branches and nightly build.

Author: SylvainDe

.travis.yml

@@ -4,6 +4,10 @@ python:
   - "3.3"
   - "3.4"
   - "3.5"
+  - "3.6"
+  - "3.6-dev"
+  - "3.7-dev"
+  - "nightly"
 install: 
   - if [ "$TRAVIS_PYTHON_VERSION" == "3.2" ]; then travis_retry pip install coverage==3.7.1; fi
   - if [ "$TRAVIS_PYTHON_VERSION" != "3.2" ]; then travis_retry pip install coverage; fi