TST fix Travis build for Python 3.5

By default there is no Python 3.5 installed on Travis,
but Travis installs it if we ask it to run tox using Python 3.5

.travis.yml

@@ -1,5 +1,5 @@
 language: python
-python: 2.7
+python: 3.5
 
 env:
 - TOXENV=py26 LUA=libluajit-5.1-dev