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 +4,11 @@ python:
   - "2.7"
   - "3.3"
   - "3.4"
+  - "3.5"
+  - "3.6"
+  - "3.6-dev"
+  - "3.7-dev"
+  - "nightly"
 install:
   - pip install -r requirements.txt
   - pip install codecov pytest-cov