Update .travis.yml

.travis.yml

@@ -1,6 +1,10 @@
 language: python
-python:
+# Enable python 3.7 without globally enabling sudo and dist: xenial for other build jobs
-  - "3.7"
+matrix:
+  include:
+    - python: 3.7
+      dist: xenial
+      sudo: true
 install:
   - pip install pipenv
   - pipenv install --dev