Add cover to Travis CI build matrix
diff --git a/.travis.yml b/.travis.yml
index 1a63f343f7d..c444081d9f8 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -20,3 +20,4 @@ script: travis_retry tox
 env:
   - TOXENV=py${TRAVIS_PYTHON_VERSION//[.]/}
   - TOXENV=lint
+  - TOXENV=cover
