Constrain rubyforge specification.

- When building a gem that depends on the rubyforge plug-in, the warning below is produced by RubyGems:

```
    WARNING:  open-ended dependency on rubyforge (>= 2.0.4, development) is not recommended
      if rubyforge is semantically versioned, use:
        add_development_dependency 'rubyforge', '~> 2.0', '>= 2.0.4'
    WARNING:  See http://guides.rubygems.org/specification-reference/ for help
```

This change silences this warning.