Use github url as a gem's homepage

[kyanny]

Hi, I suggest to change the homepage url written at gemspec file.

I think the github project url is useful because of some reasons.

• Developers can find your repo and easy to contribute
• gem-src integration will be available

Either you can define homepage_uri or soruce_code_uri at edit gem links page at rubygems.org. It also enable to integrate with gem-src. https://rubygems.org/gems/nyan-cat-formatter/edit (requires login to rubygems.org)