Fixture should depend on MockerCore

MockerCore has all the functions like start and the proxying functions
to adapters that fixtures needs. Mocker includes things like context
manager which it doesn't.

requests_mock/contrib/fixture.py

@@ -15,11 +15,11 @@ import fixtures
 from requests_mock import mocker
 
 
-class Fixture(fixtures.Fixture, mocker.Mocker):
+class Fixture(fixtures.Fixture, mocker.MockerCore):
 
     def __init__(self, **kwargs):
         fixtures.Fixture.__init__(self)
-        mocker.Mocker.__init__(self, **kwargs)
+        mocker.MockerCore.__init__(self, **kwargs)
 
     def setUp(self):
         super(Fixture, self).setUp()