O Vagrant estava coaxando após a atualização para 1.5. O erro:
$ vagrant up
Vagrant failed to initialize at a very early stage:
The plugins failed to load properly. The error message given is
shown below.
undefined method `register_middleware' for #<Faraday::Connection:0x00000100ba8780>
O conserto:
$ sudo /Applications/Vagrant/embedded/bin/gem uninstall faraday
$ sudo /Applications/Vagrant/embedded/bin/gem install faraday --version 0.8.5
Gorjeta de chapéu para Berkshelf Issue # 979 e Vagrant Issue # 3128