Log Message:
Update ruby-cassandra to 0.15.0.

v0.15.0
- Fixes for ruby 1.8 (issue #158, courtesy @jarib)
- get_indexed_slices returns an OrderedHash (issue #110)

v0.14.0
- Numerous performance improvements (courtesy @nearbuy)
- Fixed many 0.6 bugs
- Added batch counter update support to Cassandra::Mock
- Add support DynamicComposite columns (issue #154, courtesy @nearbuy)
- API cleanup in get_indexed_slices (issue #155, courtesy @mcmire)