Vagrant, use nfs if available

If you're developing a project both on windows and Linux / Mac you can enable nfs only if the platform supports the protocol:

if RUBY_PLATFORM.downcase.include?("darwin") or RUBY_PLATFORM.downcase.include?("linux")
    config.vm.synced_folder ".", "/vagrant", :nfs => true
    else
        config.vm.synced_folder ".", "/vagrant"
end