gemspec: Drop defunct rubyforge_project directive (#74)
gemspec: Drop defunct rubyforge_project directive (#74)

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

Background: 

- RubyForge was closed down in 2013, https://twitter.com/evanphx/status/399552820380053505
- rubygems/rubygems#2436 deprecated the `rubyforge_project` property
savonrb/gyoku