When running a bundle install on pre-1.3 versions of Bundler, it rejects the :ruby_20 symbol used in platform constraints:

/home/dcleal/.rvm/gems/ruby-1.8.7-p371@global/gems/bundler-1.1.4/lib/bundler/dsl.rb:220:in `_normalize_options': `ruby_20` is not a valid platform. The available options are: [:rbx, :ruby_18, :jruby, :ruby_19, :mswin, :mri, :mingw, :mri_18, :mingw_18, :mri_19, :ruby, :mingw_19]