Revert r42942; a revert requires a reason

* rational.c (nurat_div): reverted r28844, r28886 and r28887.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42952 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2013-09-17

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42953 b2dd03c8-39d4-4d8f-98ff-823fe69b080e