Add 3.6-dev to .travis.yml (#316)

diff --git a/.travis.yml b/.travis.yml
index f1391e2..a6c6d83 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -2,6 +2,7 @@
 language: python
 python:
   - "nightly"
+  - "3.6-dev"
   - "3.5"
   - "3.4"
   - "3.3"