Vagrantfile: explicitly specify rsync as the shared folder driver

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>
(cherry picked from commit f8f0524)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Vagrantfile

@@ -30,6 +30,8 @@ Vagrant.configure("2") do |config|
     v.cpus = cpus
   end
 
+  config.vm.synced_folder ".", "/vagrant", type: "rsync"
+
   # Disabled by default. To run:
   #   vagrant up --provision-with=upgrade-packages
   # To upgrade only specific packages: