Test on Python 3.7

Previously it was being tested in the development environment for Python 3.7 on Travis CI, now it's running in the officially supported environment.

Fast finish was removed since it doesn't do anything when the matrix is so small.

Author: kevin-brown

.travis.yml

@@ -1,11 +1,9 @@
 language: python
+dist: xenial
 
 python:
   - "3.6"
-  - "3.7-dev"
-
-matrix:
-  fast_finish: true
+  - "3.7"
 
 install: pip install tox-travis
 script: tox