Add Vagrant version upper bound of "< 1.8.3"

A recent PR to Vagrant, which is in released 1.8.3+, breaks our usage of
`minion_id` to pass additional args to Salt.
See hashicorp/vagrant#7207 for the PR.

Also add comments in the Vagrantfile about the version requirements.

Vagrantfile

@@ -17,7 +17,11 @@ def is_salt_master(id)
   !id.match(/servo-master\d+/).nil?
 end
 
-Vagrant.require_version '>= 1.8.0'
+# Need Vagrant >= 1.8.0, in which the Vagrant Salt provisioner was overhauled
+# See https://github.com/servo/saltfs/pull/180
+# Vagrant 1.8.3+ breaks our usage of minion_id to pass additional args to Salt
+# See https://github.com/mitchellh/vagrant/pull/7207
+Vagrant.require_version('>= 1.8.0', "< 1.8.3")
 
 Vagrant.configure(2) do |config|