Add vagrant-digitalocean to the 'plugins' group and update the tag from which vagrant is used

This change allows you to `bundle exec vagrant --provider=digital_ocean`. Without the plugin being
in the 'plugins' group, it's difficult to test the plugin directly from the repo. See
https://docs.vagrantup.com/v2/plugins/packaging.html for more information.
Merge pull request #119 from skottler/gem_group

Add vagrant-digitalocean to the 'plugins' group and update the tag from ...