Log Message:
Update ruby-cassandra to 0.17.0.

v0.17.0
- multi_get_columns with only one query (courtesy @christian-blades-cb)
- documentation fixes for get and multi_get (issue #136, courtesy \ 
@christian-blades-cb)

v0.16.0
- New :queue_size option for batch mode (courtesy @vicentllongo)
- Cassandra 1.1 support (courtesy @hawknewton)