Add reference manual of Set#select!

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 #50 from monkey-mas/add-reference-manual-of-Set-select!

Add reference manual of Set#select!