fix guard for recent rubies"

Gemfile

@@ -17,6 +17,6 @@ end
 
 group :guard do
   gem "growl"
-  gem "guard-rspec", platforms: [:ruby_19, :ruby_20, :ruby_21, :ruby_22]
+  gem "guard-rspec", platforms: [:ruby_19, :ruby_20, :ruby_21, :ruby_22, :ruby_23, :ruby_24]
   gem "rb-fsevent"
 end