undefined method `t` for WalkSpec class

Hey,

On line 41 of https://github.com/basho/riak-ruby-client/blob/master/lib/riak/walk_spec.rb#L41 we call the method `t` which is not defined.  In order to use `t` in the class method we should also `extend Util::Translation` in addition to including it.  

I'd provide a pull request but I'm lazy.  You're welcome to assign the issue to me and I'll fix it after Christmas.

Thanks All!