Python 3.7 build added to .travis.yml

Author: jia3ep

.travis.yml

@@ -4,8 +4,8 @@ python:
      - "3.3"
      - "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
 # command to install dependencies
 install:
      # Coveralls 4.0 doesn't support Python 3.2