Remove incorrectly implemented part of RemoveCircularReferences

It's impossible to have a circular object reference from a deserialized JSON string - this was a remnant of the previous implementation. This check is unnecessary (and should have been calling "json_hash.reduce" anyway).
Improve and add test for Rake task integration

+Simplify README's explanation of integrations
+Make Rake integration automatic
+Remove TODO (it has an issue now)

Monkey patching #execute isn't as nice as patching #display_error_message, so we'll do that instead. I think monkey patching #execute may also cause Raven to not catch errors called with Rake's #invoke method.