see #33: downgrade to python 3.6 since 3.7 is not available at TravisCI

.travis.yml

@@ -3,7 +3,7 @@ matrix:
   include:
   - python: '2.7'
     env: ANSIBLE_VERSION=2.4.6
-  - python: '3.7'
+  - python: '3.6'
     env: ANSIBLE_VERSION=2.7.5
 install:
 - pip install ansible==$ANSIBLE_VERSION