Switch Travis-CI to use Python 3.5.1; add 3.6-dev too (#2368)

We now run 4 builds instead of 3.

If that's too much, consider dropping 3.4 (it should be covered by 3.3 and 3.5).

Author: gvanrossum

.travis.yml

@@ -3,7 +3,9 @@ language: python
 python:
   - "3.3"
   - "3.4"
-  - "3.5"
+  # Specifically request 3.5.1 because we need to be compatible with that.
+  - "3.5.1"
+  - "3.6-dev"
   # Pypy build is disabled because it doubles the travis build time, and it rarely fails
   # unless one one of the other builds fails.
   # - "pypy3"