Fix Vagrantfile for 2.0.3+

With Vagrant 2.0.3 and up, the DEFAULT_SERVER_URL is frozen, so it cannot be changed. But earlier versions of Vagrant need this line to work. While we could mandate a minimum Vagrant version, this change would set the correct URL if it's wrong, and leave it alone if the Vagrant version is new enough not to need it.

In hashicorp/vagrant#9442 this fix was posted by @reedy, credit where credit is due.

vagrant-spk

@@ -59,7 +59,9 @@ VM_NAME = File.basename(File.dirname(File.dirname(__FILE__))) + "_sandstorm_#{Ti
 VAGRANTFILE_API_VERSION = "2"
 
 # ugly hack to prevent hashicorp's bitrot. See https://github.com/hashicorp/vagrant/issues/9442
-Vagrant::DEFAULT_SERVER_URL.replace('https://vagrantcloud.com')
+unless Vagrant::DEFAULT_SERVER_URL.frozen?
+  Vagrant::DEFAULT_SERVER_URL.replace('https://vagrantcloud.com')
+end
 
 Vagrant.configure(VAGRANTFILE_API_VERSION) do |config|
   # Base on the Sandstorm snapshots of the official Debian 8 (jessie) box.