Removed -p switch from mkdir if communicator is WinRM

The -p switch on mkdir in sync_folders was blowing up on a box with Powershell 4. In that version, they added a new common switch called -PipelineVariable, and it causes -p to be ambiguous. I added a check on the communicator for WinRM, and removed the switch if so. It's also not needed on previous versions of Powershell since mkdir in Powershell is recursive by default (it aliases to the New-Item cmdlet), so an additional version check is unecessary.
I would attempt to write them, but I currently don't have an adequate environment to run them; I'm running Ruby 1.9.1 on Windows since that is what is bundled with the Chef client, and rake fails with syntax errors (looks like this was also mentioned at https://github.com/mitchellh/vagrant/issues/2692).

It appears the Chef client may run on Ruby 2, but they don't officially support it yet. I'll see if I can update and run the tests successfully.