If using from sources, ruby2 is needed

In OS packages ruby2 comes embedded, but if you install vagrant from sources, you should execute vagrant with ruby2.

At least, with ruby 1.9.1 and using docker as a provisioner, it gives the error:

/var/lib/gems/1.9.1/gems/vagrant-1.4.1.dev/plugins/provisioners/docker/plugin.rb:13:in `require_relative': /var/lib/gems/1.9.1/gems/vagrant-1.4.1.dev/plugins/provisioners/docker/config.rb:23: syntax error, unexpected tPOW (SyntaxError)
      def run(name, **options)