Enable unit tests on Python 3.5

This enables the tests in tox and in Travis.  This ought to prevent
regressions in Python 3 support.

Note that Python 3.4 isn't supported (yet) because the vault code relies on
%-formatting of byte strings, a feature that was ripped out of Python 3.0
and restored in 3.5.

Also note that passing tests don't mean that Ansible itself can already
be used with Python 3.5: test coverage is only 53%.

tox.ini

@@ -10,4 +10,4 @@ commands =
     py27: python -m compileall -fq -x 'test|samples' lib test contrib
     py{34,35}: python -m compileall -fq -x 'lib/ansible/module_utils|lib/ansible/modules' lib test contrib
     # Unittests need lots of work to make code python3 compatible
-    py{26,27}: make tests
+    py{26,27,35}: make tests