Update our list of travis python environments

Travis CI's pypy3 is very old and breaks when we try to use setuptools
with it. Python 3.6 is something we should start testing on.

.travis.yml

@@ -19,10 +19,10 @@ matrix:
       python: 3.4
     - env: TOXENV=py35
       python: 3.5
+    - env: TOXENV=py36
+      python: 3.6
     - env: TOXENV=pypy
       python: pypy
-    - env: TOXENV=pypy3
-      python: pypy3
     - env: TOXENV=py27-flake8
       python: 2.7
     - env: TOXENV=py34-flake8