Add Python 3.6 (#214)

* Add Python 3.6

Plus the other explicit major.minor versions supported
diff --git a/.travis.yml b/.travis.yml
index 8a9d04d..0fd8463 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -5,4 +5,5 @@
   - "3.3"
   - "3.4"
   - "3.5"
+  - "3.6"
 script: python tests/all_tests.py