Use Xenial for Py 3.6.

.travis.yml

@@ -3,7 +3,11 @@ language: python
 matrix:
     include:
     - python: 3.6
+      dist: xenial
+      sudo: true
     - python: 3.7
+      dist: xenial
+      sudo: true
 
 install:
   - pip install Cython==0.29 cymem