Require at least Vagrant 1.2.x in Vagrantfile

Users don't read READMEs or documentation, they'll
just fire 'vagrant up'. Vagrant already checks the
Virtualbox version itself, but for the Vagrant version
no checks exist. Vagrant 1.4 provides a method 'require_version'
which is pretty useless for older versions, like on Debian
Wheezy (1.0.3). So we'll ship our own version comparison.

fixes #6366

Vagrantfile

@@ -1,7 +1,20 @@
 # -*- mode: ruby -*-
 # vi: set ft=ruby :
 
-Vagrant.configure("2") do |config|
+VAGRANTFILE_API_VERSION = "2"
+VAGRANT_REQUIRED_VERSION = "1.2.0"
+
+# Require 1.2.x at least
+if ! defined? Vagrant.require_version
+  if Gem::Version.new(Vagrant::VERSION) < Gem::Version.new(VAGRANT_REQUIRED_VERSION)
+    puts "Vagrant >= " + VAGRANT_REQUIRED_VERSION + " required. Your version is " + Vagrant::VERSION
+    exit 1
+  end
+else
+  Vagrant.require_version ">= " + VAGRANT_REQUIRED_VERSION
+end
+
+Vagrant.configure(VAGRANTFILE_API_VERSION) do |config|
   # All Vagrant configuration is done here. The most common configuration
   # options are documented and commented below. For a complete reference,
   # please see the online documentation at vagrantup.com.