BLD: Enable 3.7 for real.

Author: Scott Sanderson

Diff for: .travis.yml

@@ -5,7 +5,12 @@ python:
   - "3.4"
   - "3.5"
   - "3.6"
-  - "3.7"
+
+matrix:
+  include:
+  - python: 3.7
+    dist: xenial
+    sudo: true
 
 before_script:
   - pip install tox