Infrastructure changes

* Use new and shiny require_relative (rspec/rspec-expectations#476 (comment))
* 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