Remove use of Tilt::Cache

[jeremyevans]

I'm planning on deprecating Tilt::Cache in the next release of tilt (2.2.0), as it isn't used internally, isn't thread safe, and doesn't add any real value (faster to use a plain hash). grape-rabl should either copy the Tilt::Cache implementation, or switch to using a plain hash, or if you really need API compatibility for the #fetch method:

class Cache < Hash
  def fetch(*args)
    super(args) do
      self[args] = yield
    end
  end
end