Vagrantfile: Use 2 virtual CPUs with virtualbox.

This should improve performance of linting and other parallelizable
operations in the development environment on most laptops.

Vagrantfile

@@ -67,6 +67,7 @@ Vagrant.configure(VAGRANTFILE_API_VERSION) do |config|
     override.vm.box = "ubuntu/trusty64"
     # It's possible we can get away with just 1.5GB; more testing needed
     vb.memory = 2048
+    vb.cpus = 2
   end
 
 $provision_script = <<SCRIPT