Riak 2.0 support

This PR is to get started with Riak 2.0 support:

1. It adds `updateDtype` and `fetchDtype` methods common for all CRDTs in Riak 2.0.

More information on CRDTs in Riak available here:
https://gist.github.com/russelldb/7316f83ddd38965d9f76
https://github.com/basho/riak/issues/354

2. Updates Travis build to use Riak 2.0 (2.0.0pre11 at this moment)

3. Yokozuna support yet to come so search tests are disabled

Make sure you run `make dt-setup` before running tests. This is because at present you can only manually create and activate new bucket types: https://github.com/basho/riak/issues/484