Update the `warn` method signature to support Rails 3.1

[toppa]

Description

I encountered this error after adding next_rails to the project we're using in the Rails upgrade workshop. This error did not happen before adding next_rails

I traced it to a warn call in a gem being used by the project. The warn call in Rails 3 has a category keyword argument that our monkey patch in next_rails is not supporting https://github.com/countries/country_select/blob/7e73c4cc465aa61c268867b985837f2a126cffdc/lib/country_select/country_select_helper.rb#L10

How Has This Been Tested?

Tested locally. Making this change resolves the error.

I will abide by the code of conduct

[toppa]

@arielj or @rishijain please review when you have a minute. I'm hoping to have this released for RailsConf next week.