Vagrantfile: relax Salt pinned version to 2019.2

Fixes provisioning of a new VM.

Old versions like 2019.2.3 may be moved to an archive
and get an HTTP 404 error.
Relaxing the pinned version allows setup to find
newer patches, such as 2019.2.5.

More info:
752768b1ff/accepted/0022-old-releases.md

Vagrantfile

@@ -25,7 +25,7 @@ Vagrant.configure(VAGRANT_CONFIG_VERSION) do |config|
       salt.log_level = "info"
 
       salt.install_type = "stable"
-      salt.version = "2019.2.3"
+      salt.version = "2019.2"
   end
 
   config.vm.provider "virtualbox" do |vb|