prepend private_class_method to private class methods

this applies even if they are in the 'private part' of a class
definition as in:

class Foo
  ...
private
  def foo; 1; end
  def self.foo; 2; end
end

Foo.foo #=> 2
Foo.new.foo #=> NoMethodError: private method `foo' called
do not use ruby 2.1 features so we still support 2.0 inofficially
Merge pull request #1092 from opf/fix/ruby_2_0

Fix/ruby 2 0