Add most recent Python versions in Travis CI

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

Author: SylvainDe

.travis.yml

@@ -5,6 +5,10 @@ python:
   - "3.3"
   - "3.4"
   - "3.5"
+  - "3.6"
+  - "3.6-dev"
+  - "3.7-dev"
+  - "nightly"
   # - "pypy" - won't work as smmap doesn't work (see gitdb/.travis.yml for details)
 #matrix:
 #  allow_failures: