This goes along with the previous multiple keyspace support PR

It allows you to explicitly provide a Rails.env value to `create_keyspace!` and `drop_keyspace!`. Which allows users more flexibility in creating Rake tasks to manage their multiple keyspace configurations.
I've edited the existing Rake task to pass in the current Rails.env so the default behaviour hasn't changed.

(Also bumped the version of cql-rb, since they've released 2.0.0 final)