Add more interpreters to Tox

tox.ini

@@ -4,8 +4,9 @@
 # and then run "tox" from this directory.
 
 [tox]
-envlist = py27, py35, pypy
+envlist = py27, py34, py35, py36, pypy
 skipsdist = True
+skip_missing_interpreters = True
 
 [testenv]
 skip_install = True