Fix Python 3.7 build

Author: alanhdu

.travis.yml

@@ -1,7 +1,10 @@
 language: python
-python:
-- 3.6
-- 3.7
+matrix:
+  include:
+    - python: 3.7
+      dist: xenial
+      sudo: true
+    - python: 3.6
 
 notifications:
   email: false