add python3.7 to travis CI

Author: conradho

.travis.yml

@@ -7,6 +7,10 @@ python:
   - 3.6
   - 3.7
 
+# tmp fix until travis gets python3.7
+dist: xenial
+sudo: true
+
 install:
   - pip install -r requirements.txt
   - pip install -e .