Update the README now that Rails 4.2.beta is out
Update the README now that Rails 4.2.beta is out

and remove the no longer needed section about the I18n placeholder
change in Rails 4.2, which was reverted.

johan--/ransack,nagyt234/ransack,martndemus/ransack,avit/ransack,activerecord-hackery/ransack,Mario245/ransack,AndrewSwerlick/ransack,roman-franko/ransack,Eric-Guo/ransack,laserlemon/ransack,vaski/ransack,guilleva/ransack,reinteractive/ransack,mariorcardoso/ransack,amboxer21/ransack,outstand/ransack