Plugin winrm not needed anymore and causing error in Vagrant 2+

[CristianCantoro]

I am using vagrant 2.1.5 on Ubuntu 16.04 and if I run vagrant up I get the following error:

Installing the 'winrm' plugin. This can take a few minutes...
/opt/vagrant/embedded/gems/2.1.5/gems/vagrant-2.1.5/lib/vagrant/plugin/manager.rb:156:in `install_plugin': undefined method `name' for nil:NilClass (NoMethodError)
	from /opt/vagrant/embedded/gems/2.1.5/gems/vagrant-2.1.5/plugins/commands/plugin/action/install_gem.rb:30:in `call'
	from /opt/vagrant/embedded/gems/2.1.5/gems/vagrant-2.1.5/lib/vagrant/action/warden.rb:34:in `call'
	from /opt/vagrant/embedded/gems/2.1.5/gems/vagrant-2.1.5/lib/vagrant/action/builder.rb:116:in `call'
	from /opt/vagrant/embedded/gems/2.1.5/gems/vagrant-2.1.5/lib/vagrant/action/runner.rb:66:in `block in run'
	from /opt/vagrant/embedded/gems/2.1.5/gems/vagrant-2.1.5/lib/vagrant/util/busy.rb:19:in `busy'
	from /opt/vagrant/embedded/gems/2.1.5/gems/vagrant-2.1.5/lib/vagrant/action/runner.rb:66:in `run'
	from /opt/vagrant/embedded/gems/2.1.5/gems/vagrant-2.1.5/plugins/commands/plugin/command/base.rb:14:in `action'
	from /opt/vagrant/embedded/gems/2.1.5/gems/vagrant-2.1.5/plugins/commands/plugin/command/install.rb:70:in `block in execute'
	from /opt/vagrant/embedded/gems/2.1.5/gems/vagrant-2.1.5/plugins/commands/plugin/command/install.rb:69:in `each'
	from /opt/vagrant/embedded/gems/2.1.5/gems/vagrant-2.1.5/plugins/commands/plugin/command/install.rb:69:in `execute'
	from /opt/vagrant/embedded/gems/2.1.5/gems/vagrant-2.1.5/plugins/commands/plugin/command/root.rb:66:in `execute'
	from /opt/vagrant/embedded/gems/2.1.5/gems/vagrant-2.1.5/lib/vagrant/cli.rb:54:in `execute'
	from /opt/vagrant/embedded/gems/2.1.5/gems/vagrant-2.1.5/lib/vagrant/environment.rb:291:in `cli'
	from /opt/vagrant/embedded/gems/2.1.5/gems/vagrant-2.1.5/bin/vagrant:164:in `<main>'
Installing the 'winrm' plugin. This can take a few minutes...

I found a reference to this error in this issue: https://github.com/hashicorp/vagrant/issues/9475#issuecomment-366336176 which says:

You should not have to install the winrm plugin as it is now fully integrated into Vagrant itself.

In fact, removing the plugin from the Vagrantfile everything seems to work correctly. I am not opening a pull request because I don't know if you want to add some logic to support Vagrant 2 and previous versions.

Thank you.