rake gem: Support RubyGems 2.0.0 or later

RubyGems 2.0.0 changed API to create gem:

http://rubygems.rubyforge.org/rubygems-update/History_txt.html

> 2.0.0 / 2013-02-24
>
> * Merged Gem::Builder into Gem::Package.
>   Use Gem::Package.build instead of Gem::Builder.new(spec).build
Merge pull request #742 from kou/work-rake-gem-with-latest-rubygems

rake gem: Support RubyGems 2.0.0 or later