Add reference manual of Set.keep_if()

The reference manual of Set.keep_if() is no currently available.
According to https://github.com/ruby/ruby/tree/v1_9_2_381, this method was added as a new method to Ruby 1.9.2.
The reference manual of Ruby 1.9.2 and later versions will show the method with this change.
Merge pull request #49 from monkey-mas/add-reference-manual-of-Set-keep_if

Add reference manual of Set.keep_if()