Forcing string encoding to pass unit test in TravisCI.

Author: hcpchris

tests/utility_classes/test_buttons.py

@@ -370,5 +370,5 @@ def test_issue86_ButtonTheme_to_json():
     assert instance['states'] is not None
     assert instance['states'].hover is not None
 
-    result = str(instance.to_json())
+    result = str(instance.to_json(), encoding = 'utf-8')
     assert result == str(kwargs).replace("'", '"')