Use Rails format routing to match the RubyGems API

RubyGems uses the .json extension to specify that dependencies should be returned in JSON format (e.g., http://rubygems.org/api/v1/dependencies.json?gems=rails).  Rails Assets doesn't recognize that extension, so http://rails-assets.org/api/v1/dependencies.json?gems=rails-assets-jquery-ui responds with marshalled Ruby objects instead of JSON.