Minimal change to support CQLClient.execute returning a list of dicts.
Bump silverberg requirement.
Remove raw_response is None check and the corresponding CassBadDataError.  Also prune error test cases for now impossible conditions, and fix error tests using the old response format.
Merge pull request #223 from rackerlabs/dict-results

Minimal change to support CQLClient.execute returning a list of dicts.