Gemfile: update ruby version to match vagrant box

Using a fresh VM configured with the class Vagrantfile, I get this error:

    vagrant@rails-dev-box:/vagrant/proj1$ bundle install        
    Your Ruby version is 2.1.1, but your Gemfile specified 2.0.0

I think maybe the project is intended to use [the same version](https://github.com/rails-decal/rails-dev-box/blob/master/puppet/manifests/default.pp#L7) as installed by Vagrant?