Use github url as a gem's homepage

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](https://rubygems.org/gems/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)