Add Python 3.7 to Travis

Python 3.7 is the current stable version. It's useful to test our code
against it.

Unfortunately, Travis only allows testing against it via a hack, see

travis-ci/travis-ci#9815
travis-ci/travis-ci#9069 (comment)

Author: StevenMaude

.travis.yml

@@ -4,5 +4,11 @@ python:
  - "3.4"
  - "3.5"
  - "3.6"
+# Hack for Python 3.7; remove when Travis correctly supports it.
+matrix:
+  include:
+    - python: 3.7
+      dist: xenial
+      sudo: true
 script:
  - python setup.py test