Try to fix Travis

.travis.yml

@@ -1,6 +1,11 @@
 language: python
 python:
   - '3.7'
+matrix:
+  include:
+    - python: 3.7
+      dist: xenial
+      sudo: true
 install:
  - pip install -r requirements.txt
  - pip install -r dev-requirements.txt