Adding Github as homepage

When I'm looking at a gemspec, I can see which gems are used.
To then attempt to find where those gems are defined, I invariably go
to https://rubygems.org/gem/<name_of_gem>. Once there I would like to
see a link to the homepage of that gem. In most cases the Github
repository is adequate.
Merge pull request #82 from jeremyf/adding-link-github-project

Adding Github as homepage