Fix Ruby 2.5 incopatibility

We don't really need that second optional argument, 
as its default value is just what we need
https://ruby-doc.org/core-2.7.2/Module.html#method-i-private_method_defined-3F

Author: flvrone

lib/ruby2_keywords.rb

@@ -1,5 +1,5 @@
 class Module
-  unless private_method_defined?(:ruby2_keywords, true)
+  unless private_method_defined?(:ruby2_keywords)
     private
     # call-seq:
     #    ruby2_keywords(method_name, ...)