Use optimized node-cassandra-cql fork for now

Use the fork directly from github until
https://github.com/jorgebay/node-cassandra-cql/pull/103 is merged.

The fork improves throughput for larger (multi-MB) blobs by about 2-3x.
Example: [[en:Barack Obama]] pulls 1.9Gbit without & 5GBit with this fork.