rake: fix publish order

Stop a old URL of Rabbit Slide Show.

before: 1.rubygems 2.slideshare 3.speaker_deck
after: 1.slideshare 2.speaker_deck 3.rubygems
rake gem: fix NameError for RubyGems 2.0

Changed a sentence of gem build in RubyGems 2.0.

1.8: Gem::Builder.new(spec).build
2.0: Gem::Package.build(spec)
indent