Add Thomas's workaround to enable Python 3.7 testing

Author: rpep

.travis.yml

@@ -2,11 +2,18 @@ language: python
 
 python:
   - "nightly"
-  - "3.7"
   - "3.6"
   - "3.5"
   - "2.7"
+
+# Python 3.7 currently (early 2019) needs a newer Ubuntu version than the
+# default image on Travis.
 matrix:
+  include:
+    - python: 3.7
+      dist: xenial
+      sudo: true
+
   allow_failures:
     - python: "nightly"