Update README with DevKit req. for Ruby 2.x
Update README with DevKit req. for Ruby 2.x

Both Ruby 2.0.x and 2.1.x are built using a different version of DevKit than the default one.

Modify README.txt to reflect this.

Closes #231
shirosaki/rubyinstaller,shirosaki/rubyinstaller,jahanzaibbahadur/rubyinstaller,zorosteven/rubyinstaller,dreamauya/rubyinstaller,jahanzaibbahadur/rubyinstaller,modulexcite/rubyinstaller,haonaturel/rubyinstaller,oneclick/rubyinstaller,auyaauya/rubyinstaller,dreamauya/rubyinstaller,Rugbyman21/rubyinstaller,leonhooijer/rubyinstaller,jahanzaibbahadur/rubyinstaller,oneclick/rubyinstaller,vemmaverve/rubyinstaller,suvarnaraju/rubyinstaller,suvarnaraju/rubyinstaller,haonaturel/rubyinstaller,vemmaverve/rubyinstaller,zorosteven/rubyinstaller,auyaauya/rubyinstaller,jaym/rubyinstaller,zorosteven/rubyinstaller,shirosaki/rubyinstaller,leonhooijer/rubyinstaller,haonaturel/rubyinstaller,dreamauya/rubyinstaller,Rugbyman21/rubyinstaller,modulexcite/rubyinstaller,jaym/rubyinstaller,suvarnaraju/rubyinstaller,jaym/rubyinstaller,vemmaverve/rubyinstaller,modulexcite/rubyinstaller,oneclick/rubyinstaller,leonhooijer/rubyinstaller,auyaauya/rubyinstaller,Rugbyman21/rubyinstaller