chore(Vagrantfile): update to CoreOS 310.1.0

Necessary to bump like this until we can reliably switch back
to the alpha release (see #928).

Author: carmstrong

Vagrantfile

@@ -1,8 +1,6 @@
 # -*- mode: ruby -*-
 # # vi: set ft=ruby :
 
-require_relative 'contrib/coreos/override-plugin.rb'
-
 DEIS_NUM_INSTANCES = (ENV['DEIS_NUM_INSTANCES'].to_i > 0 && ENV['DEIS_NUM_INSTANCES'].to_i) || 1
 
 if DEIS_NUM_INSTANCES == 1
@@ -14,11 +12,11 @@ else
 end
 
 Vagrant.configure("2") do |config|
-  config.vm.box = "coreos-298.0.0"
-  config.vm.box_url = "http://storage.core-os.net/coreos/amd64-usr/298.0.0/coreos_production_vagrant.box"
+  config.vm.box = "coreos-310.1.0"
+  config.vm.box_url = "http://storage.core-os.net/coreos/amd64-usr/310.1.0/coreos_production_vagrant.box"
 
   config.vm.provider :vmware_fusion do |vb, override|
-    override.vm.box_url = "http://storage.core-os.net/coreos/amd64-usr/298.0.0/coreos_production_vagrant_vmware_fusion.box"
+    override.vm.box_url = "http://storage.core-os.net/coreos/amd64-usr/310.1.0/coreos_production_vagrant_vmware_fusion.box"
   end
 
   config.vm.provider :virtualbox do |vb, override|