def add_development_dependency(gem, *requirements)
    add_dependency_with_type(gem, :development, *requirements)
  end
#
Adds a development dependency named +gem+ with +requirements+ to this
Gem.  For example:

spec.add_development_dependency 'example', '~> 1.1', '>= 1.1.4'

Development dependencies aren't installed by default and aren't
activated when a gem is required.