Improve the #inspect formatting of Racecar::Message

Messages now display the topic, partition, and offset when inspected.

lib/racecar/message.rb

@@ -21,5 +21,9 @@ def create_time
     def ==(other)
       @rdkafka_message == other.instance_variable_get(:@rdkafka_message)
     end
+
+    def inspect
+      "#{topic}/#{partition}:#{offset}"
+    end
   end
 end