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.