IronRuby: Add support of Rational with floating point or real values

_From @Taritsyn on August 24, 2014 11:21_

The current version of the [`rational18.rb`](https://github.com/IronLanguages/main/blob/master/Languages/Ruby/StdLib/ruby/1.9.1/rational18.rb) does not support of Rational with floating point or real values.

This feature is needed to work with Sass version 3.4.X.

_Copied from original issue: IronLanguages/main#215_
