downgrade coverage dependency (#394)

Author: Elliot Boschwitz

requirements-dev.txt

@@ -5,7 +5,7 @@ future >= 0.16.0
 setuptools >= 36.0.1
 requests >= 2.13.0
 wheel >= 0.29.0
-coverage >= 4.3.4
+coverage >= 4.3.4, < 5.0
 twine >= 1.11.0
 bumpversion >= 0.5.3 
 tox >= 2.7.0