Fix the second-choice logger

lib/sharp-coin/logging.rb

@@ -21,7 +21,7 @@ module SharpCoin
     end
     
     def logger
-      @logger || self.class.logger || DummyLogger.new 
+      @logger || Logging::logger || DummyLogger.new 
     end
     
     # @param[Array[Symbol,String]|Array[String]] Data to log. Can specify a log