Gemspec cleanup

I audited the fields used in the gemspec and updated things according to the current gemspec reference: http://guides.rubygems.org/specification-reference/

Comments inline.
rubygems 1.2.0 was [released in 2008-06-21](https://github.com/rubygems/rubygems/blob/master/History.txt#L2143), past the EOL for rubies we're supporting.