Use xenial dist in Travis CI (#78)

🍝 Reading this (https://docs.travis-ci.com/user/languages/python/#development-releases-support), trusty has outdated Python versions. Python3.7-dev in there was really Python 3.7.0a4 instead of the latest release. With xenial, `3.7` gives us Python 3.7.1, and `3.7-dev` gives us Python 3.7.2+.
gidgethub · Feb 13, 2019 · 24feb6c · 24feb6c
1 parent ea15881
commit 24feb6c
Showing 1 changed file with 3 additions and 1 deletion.
diff --git a/.travis.yml b/.travis.yml
@@ -1,9 +1,11 @@
 language: python
 cache: pip
 python:
-  - "3.6-dev"
+  - "3.6"
+  - "3.7"
   - "3.7-dev"
   - "nightly"
+dist: xenial
 
 matrix:
   fast_finish: true