Gem infrastructure changes
* Remove references to RubyForge as it will die soon
* Require Ruby >= 1.9.3 and build on newer rubies
* Remove bundler_stubis from gitignore as Google knows nothing about it
* Remove references to RDoc as project uses YARD
* Remove rubygems_version as it shouldn't be specified (http://guides.rubygems.org/specification-reference/#rubygems_version)