Gemspec: Drop EOL'd property rubyforge_project

[olleolleolle]

The RubyGems gemspec property rubyforge_project has been removed without a replacement.

[olleolleolle]

I made a second commit, to make the CLA bot re-run. Changing homepage is not done using the gemspec's homepage attribute any longer, it's a part of metadata: { "homepage_uri" => "..." }