0.5.0 Handle inheriting from builtins

There is an issue with the previous version, which wouldn't correctly handle the following case:

class CustomClass(dict):
    def __json__(self):
        return "testing"

See this established post for more info about the problem:
https://stackoverflow.com/questions/16405969/how-to-change-json-encoding-behaviour-for-serializable-python-object/16406798#16406798

The 0.5.0 release fixes this, and should maintain backwards compatibility