Make it work with Rails 4 and Ruby 2

Thank you for writing this gem, it's my favorite for the job and would like to keep using it now that Rails 4 is out.

There aren't any significant changes except in the last commit, where I removed the `options`.  It doesn't seem to be an easy way to preserve this functionality, you can see how Rails 4 implements it here https://github.com/rails/activerecord-deprecated_finders/blob/master/lib/active_record/deprecated_finders/relation.rb#L12   I love `slugged` because its simplicity and this makes it even simpler.  In addition, there are no tests that use this argument, so nothing breaks.