Update Travis Python 3.7 build (carltongibson#997)

* Python version numbers should be strings

* Add updated Python 3.7 build

* Travis released Xenial support

* Update utility builds to Python 3.7

* Enable 'fast_finish' for Travis

.travis.yml

@@ -1,11 +1,12 @@
 sudo: false
 
+dist: xenial
 language: python
 python:
   - "3.4"
   - "3.5"
   - "3.6"
-  - "3.7-dev"
+  - "3.7"
 
 cache: pip
 
@@ -18,10 +19,11 @@ script:
   - detox
 
 matrix:
+  fast_finish: true
   include:
-    - python: 3.6
+    - python: "3.7"
       env: TOXENV=isort,lint,docs
-    - python: 3.6
+    - python: "3.7"
       env: TOXENV=warnings
   allow_failures:
     - env: TOXENV=warnings