Add travis testing for python 3.7 and 3.8 and remove workaround
diff --git a/.travis.yml b/.travis.yml
index 6d5f9a5..d71276e 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -2,12 +2,6 @@
 python:
   - "2.7"
   - "3.6"
-
-# Enable 3.7 without globally enabling sudo and xenial for other build jobs
-# See https://github.com/travis-ci/travis-ci/issues/9815
-matrix:
-  include:
-    - python: 3.7
-      dist: xenial
-      sudo: true
+  - "3.7"
+  - "3.8"
 script: python tests/all_tests.py