Further update Gemfile groups for Vagrant v1.5

Expands on 7815ee7, preventing errors and warnings on the `bundle exec rake test` (e.g., `Unknown configuration section 'omnibus'` and `Your Gemfile lists the gem vagrant-digitalocean (>= 0) more than once`).

Vagrant's [v1.5 blog post](https://www.vagrantup.com/blog/vagrant-1-5-plugin-improvements.html#toc_1) suggests that gone are the days when "plugin developers had to create Vagrantfiles that manually did a require of all plugins they needed" (also in [documentation](https://docs.vagrantup.com/v2/plugins/packaging.html)). 

However, the vagrant-aws plugin (that the vagrant docs so often cite as *the* model) currently still uses the old method of requiring vagrant-aws at top of Vagrantfile, leaving [some](https://github.com/mitchellh/vagrant-aws/pull/246#issuecomment-50064189) asking "What's the proper way to load our local fork in the Vagrant bundler environment?"

I don't understand it all but went with what appears to be the more up-to-date approach. [Vagrant-parallels](https://github.com/Parallels/vagrant-parallels/blob/307b69b1d2e0d195315b85a1563180430277a66a/Gemfile) is one of the few [provider plugins](https://github.com/mitchellh/vagrant/wiki/Available-Vagrant-Plugins#providers) to do the same. I'm guessing the others just haven't gotten around to it.
I removed the version tag for vagrant, similar to [vagrant documentation](https://docs.vagrantup.com/v2/plugins/packaging.html) and [vagrant-aws](https://github.com/mitchellh/vagrant-aws/commit/eda00c367c3d1d2e7e50ca5a4442de8ed398031f#diff-8b7db4d5cc4b8f6dc8feb7030baa2478). Maybe there is a reason to keep it.