follow vagrant plugin guide for default values

With Vagrant 1.6.1 and vagrant-berkshelf 2.0.1 I am consistently getting the following error whenever I have a project with a Berksfile path that is not set to the default.  I have not tried this with the default Berksfile path.  
```
/home/benkeith/.vagrant.d/gems/gems/vagrant-berkshelf-2.0.1/lib/berkshelf/vagrant/action/install.rb:52:in `warn_disabled_but_berksfile_exists': undefined method `berkshelf' for nil:NilClass (NoMeth
odError)
        from /home/benkeith/.vagrant.d/gems/gems/vagrant-berkshelf-2.0.1/lib/berkshelf/vagrant/action/install.rb:20:in `call'
        from /opt/vagrant/embedded/gems/gems/vagrant-1.6.1/lib/vagrant/action/warden.rb:34:in `call'
        from /opt/vagrant/embedded/gems/gems/vagrant-1.6.1/lib/vagrant/action/builtin/provision.rb:80:in `call'
        from /opt/vagrant/embedded/gems/gems/vagrant-1.6.1/lib/vagrant/action/warden.rb:34:in `call'
        from /opt/vagrant/embedded/gems/gems/vagrant-1.6.1/plugins/providers/virtualbox/action/check_accessible.rb:18:in `call'
        from /opt/vagrant/embedded/gems/gems/vagrant-1.6.1/lib/vagrant/action/warden.rb:34:in `call'
        from /opt/vagrant/embedded/gems/gems/vagrant-1.6.1/lib/vagrant/action/warden.rb:95:in `block in finalize_action'
        from /opt/vagrant/embedded/gems/gems/vagrant-1.6.1/lib/vagrant/action/warden.rb:34:in `call'
        from /opt/vagrant/embedded/gems/gems/vagrant-1.6.1/lib/vagrant/action/warden.rb:34:in `call'
        from /opt/vagrant/embedded/gems/gems/vagrant-1.6.1/lib/vagrant/action/builder.rb:116:in `call'
        from /opt/vagrant/embedded/gems/gems/vagrant-1.6.1/lib/vagrant/action/runner.rb:66:in `block in run'
        from /opt/vagrant/embedded/gems/gems/vagrant-1.6.1/lib/vagrant/util/busy.rb:19:in `busy'
        from /opt/vagrant/embedded/gems/gems/vagrant-1.6.1/lib/vagrant/action/runner.rb:66:in `run'
        from /opt/vagrant/embedded/gems/gems/vagrant-1.6.1/lib/vagrant/action/builtin/call.rb:53:in `call'
        from /opt/vagrant/embedded/gems/gems/vagrant-1.6.1/lib/vagrant/action/warden.rb:34:in `call'
        from /opt/vagrant/embedded/gems/gems/vagrant-1.6.1/lib/vagrant/action/warden.rb:95:in `block in finalize_action'
```

I have traced through it a bunch and the problem is that when vagrant-berkshelf makes the check for enabled at https://github.com/Quoin/vagrant-berkshelf/blob/master/lib/berkshelf/vagrant/action/install.rb#L18 it is always returning false, even when I have explicitly put `config.berkshelf.enabled = true` in my Vagrantfile.  Apparently this is due to this plugin not following the proper Vagrant plugin configuration convention for default values (http://docs.vagrantup.com/v2/plugins/configuration.html), since once I change it to follow that guide, it works fine.  I have the problem with both multiple machines as well as single machine setups.