[EH/GI] [71317016] Lock riak-client to latest possible version

Version 1.4.4 is compatible with riak-server 1.3.1 and doesn't require
code changes in seo_fusion (or other riak dependent services).  This is the first step in upgrading AG's riak-client dependency.

https://www.pivotaltracker.com/story/show/71317016

cc @martinstreicher FYI