FindRuby: Add support for Ruby 2.0 and 2.1

The most significant change since Ruby 1.9 is the changed path of
config.h, from RbConfig::CONFIG['rubyhdrdir'] to
RbConfig::CONFIG['rubyarchhdrdir'].
Help: Add FindRuby-2 topic release notes
Merge topic 'FindRuby-2' into next

57abfeaa Help: Add FindRuby-2 topic release notes
165f6430 FindRuby: Add support for Ruby 2.0 and 2.1