Try to fix Python 3.7 in Travis

travis-ci/travis-ci#9815 (comment)

Author: sastred

.travis.yml

@@ -1,4 +1,7 @@
 language: python
+python: 3.7
+dist: xenial
+sudo: true
 matrix:
   include:
   - env: TOXENV=py27-dj19
@@ -30,9 +33,9 @@ matrix:
   - env: TOXENV=py37-dj20
     python: 3.7
   - env: TOXENV=py35-dj21
-    python: 3.7
+    python: 3.5
   - env: TOXENV=py36-dj21
-    python: 3.7
+    python: 3.6
   - env: TOXENV=py37-dj21
     python: 3.7
 install: