Update Python versions in build matrix.

Author: Omer Katz

.travis.yml

@@ -5,8 +5,8 @@ language: python
 python:
   - "3.4"
   - "3.5"
-  - "3.5-dev" # 3.5 development branch
-  - "nightly" # currently points to 3.6-dev
+  - "3.6"
+  - "nightly" # currently points to 3.7-dev
 addons:
   postgresql: "9.3"
 services: