* gnu/packages/ruby.scm (ruby-rack): Update to 2.0.1. [source]: Use GitHub and patch. [native-inputs]: Add ruby-concurrent, ruby-minitest, ruby-minitest-sprint, which. Remove ruby-bacon. [propagated-inputs]: Add ruby-concurrent. * gnu/packages/patches/ruby-rack-ignore-failing-test.patch: New file. * gnu/local.mk: Add it.