Implement Integer#downto.
encourage people to untag rubyspecs rather than write their own tests, if possible
Raise NotImplementedError for Integer#downto with no block.
Merge pull request #387 from jerith/jerith/integer-downto

Implement Integer#downto.
Add File.unlink
Untag passing specs of File.delete
Merge pull request #391 from jstepien/file-unlink

File.unlink and File.delete