Add version constraint on yum cookbook so that it stays under v3.0

The community supported `yum` cookbook was upgraded to v3.0 which introduced some backward incompatibilities. This pull request pins our `yum` cookbook dependency to `< 3.0`.

The release of Riak 2.0 may be a good time to relax this constraint.

See also: https://github.com/basho/riak-chef-cookbook/pull/102