Use Python 3.6 instead of 3.7: latter doesn't seem to be supported by Travis

Author: epels

.travis.yml

@@ -4,7 +4,7 @@ python:
   - 'pypy3.5'
   - '2.7'
   - '3.3'
-  - '3.7'
+  - '3.6'
   - 'nightly'
 install:
   - pip install mock==2.0