RubyForge is gone

Remove references to RubyForge, as the project is defunct. Point to the http://rubydoc.info/gems/net-ldap docs instead.
Report bugs on GitHub

There seems to be issues open on GitHub, so I gambled that's the official source now that RubyForge is gone.
Merge pull request #99 from michaeltwofish/bye-rubyforge

Remove RubyForge references