🐍 revert back to python 3.6 because of travis bug

Derwent · Derwent · commit 838eaea9452d · 2019-05-05T12:46:20.000+10:00
travis-ci/travis-ci#9815
diff --git a/.travis.yml b/.travis.yml
@@ -8,7 +8,7 @@ services:
   - docker
 python:
   - "2.7"
-  - "3.7"
+  - "3.6"
   - "nightly"
 # command to install dependencies
 install:
