remove strong_parameter dependencies for rails 4 compatibility

Hi, 
  I've removed the requires for the strong_parameters gem, since it's included in Rails 4.
  I tested this on a few Rails 4 apps and it's working fine. 
  If you're interested in commiting this pull request, I think you should tag the current master head 
  as "rails_32" first, and then append  a note in the readme instructing people to use that commit
  for Rails 3.2 compatiblity.

Regards,

Sebastian.