Format project with black (#467)

Format with line length 110 to match flake8

make black part of linting check

Update travis spec for updated black requirements

Add diff output for black on failure

update changelog

tests/assertions.py

@@ -13,7 +13,7 @@ def assert_cassette_has_one_response(cass):
 
 def assert_is_json(a_string):
     try:
-        json.loads(a_string.decode('utf-8'))
+        json.loads(a_string.decode("utf-8"))
     except Exception:
         assert False
     assert True