Update rails-erd.gemspec
Update rails-erd.gemspec

Since rubyforge.org is no more, set the homepage of the project to the github repo
voormedia/rails-erd,mallikarjunayaddala/rails-erd,qiujuer/rails-erd,kerrizor/rails-erd,mallikarjunayaddala/rails-erd,sferik/rails-erd,nalabjp/rails-erd,voormedia/rails-erd,kerrizor/rails-erd,sferik/rails-erd,qiujuer/rails-erd,nalabjp/rails-erd