Add Vagrant Wrapper to Gemfile

Without this line I get this error:

```
/opt/boxen/rbenv/versions/2.0.0-p451/lib/ruby/gems/2.0.0/gems/bundler-1.6.2/lib/bundler/rubygems_integration.rb:252:in `block in replace_gem': vagrant-wrapper is not part of the bundle. Add it to Gemfile. (Gem::LoadError)
	from /opt/boxen/rbenv/versions/2.0.0-p451/bin/vagrant:22:in `<main>'
/opt/rubies/2.0.0-p451/lib/ruby/gems/2.0.0/gems/beaker-1.12.2/lib/beaker/hypervisor/vagrant.rb:151:in `block in vagrant_cmd': Failed to exec 'vagrant up' (RuntimeError)
	from /opt/rubies/2.0.0-p451/lib/ruby/gems/2.0.0/gems/beaker-1.12.2/lib/beaker/hypervisor/vagrant.rb:145:in `chdir'
```