Define respond to missing on Mock

Fixes freerange#322.

As of Ruby 2.4, using Forwardable to delegate to a missing method,
without defining respond_to_missing? on the receiver, results in
depecration warnings being shown.

Define respond_to_missing? in place of respond_to? on Mocha::Mock so
that deprecation warnings are no longer shown.

lib/mocha/mock.rb

@@ -314,7 +314,7 @@ def method_missing(symbol, *arguments, &block)
     end
 
     # @private
-    def respond_to?(symbol, include_private = false)
+    def respond_to_missing?(symbol, include_private = false)
       if @responder then
         if @responder.method(:respond_to?).arity > 1
           @responder.respond_to?(symbol, include_private)