Run Travis CI against Python 3.7 as well

.travis.yml

@@ -2,8 +2,9 @@ language: python
 
 # 3.6 is listed first so it gets used for the later build stages
 python:
-  - 3.6
-  - 3.5
+  - "3.6"
+  - "3.7-dev"
+  - "3.5"
 
 # Executed for 3.5 AND 3.5 as the first "test" stage:
 script: