Fix Python 3.7 build

Author: alanhdu

.travis.yml

@@ -3,6 +3,13 @@ python:
 - 3.6
 - 3.7
 
+matrix:
+  include:
+    - python: 3.7
+      dist: xenial
+      sudo: true
+    - python: 3.6
+
 notifications:
   email: false