Fix gem require

Replace gem dependency of 'rails' with 'railties' to reflect the modern rails-disaggregated approach. In installations where ActiveRecord isn't used and an alternative ORM/ODM is used (e.g. mongomapper/mongoid/datamapper), 'rails' is only required by this gem - a little unnecessary and heavy.

See for example jquery-fileupload-rails (https://github.com/tors/jquery-fileupload-rails/blob/master/jquery-fileupload-rails.gemspec#L21)