Log Message:
Update riak to 1.2.0.

Changes include:
* Riak supports R15 now.
* More efficiently add multiple Riak nodes to your cluster
* Stage and review, then commit or abort cluster changes for easier
  operations; plus smoother handling of rolling upgrades
* Better visibility into active handoffs
* Repair Riak KV and Search partitions by attaching to the Riak Console
  and using a one-line command to recover from data corruption/loss
* More performant stats for Riak; the addition of stats to Riak Search
* 2i and Search usage thru the Protocol Buffers API

Full release notes available here:

https://github.com/basho/riak/blob/1.2/RELEASE-NOTES.md