Retrun a boolean instead of a string

Currently `receipt.is_trial_period` returned a string instead of a boolean. I think the class should handle this.

lib/itunes/receipt.rb

@@ -65,7 +65,7 @@ def initialize(attributes = {})
         receipt_attributes[:in_app].map { |ia| self.class.new(:receipt => ia) }
       end
       @is_trial_period = if receipt_attributes[:is_trial_period]
-        receipt_attributes[:is_trial_period]
+        receipt_attributes[:is_trial_period] == "true"
       end
       @itunes_env = attributes[:itunes_env] || Itunes.itunes_env
       @latest = case attributes[:latest_receipt_info]