Drop unusupported rubyforge_project gemspec attr

Created by: olleolleolle

This PR removes support for a defunct attribute.

The RubyGems gemspec property rubyforge_project has been removed without a replacement. This PR removes that property.

Background

• RubyForge was closed down in 2013.
• rubygems/rubygems#2436 deprecated the rubyforge_project property.

Question for reviewer

• Is there anything else I should change or fix to make this work?