use vanilla rails pluck instead valium gem

since rails 3.2.1 we have vanilla method `pluck` for the same purposes as `values_of` is. In this case we can remove `valium` gem. It gives us softer dependency and looks like rails 4 support.  (Because valium gem no longer needed it doesn't support rails 4.) 

http://apidock.com/rails/v3.2.1/ActiveRecord/Calculations/pluck