Vagrant: increase default resources for VM

Without setting any defaults, it seems to give 1024MB of memory and 2
CPUs. That low of memory can (and does) result in the VM swapping itself
to death when doing some things. I'm going to set a reasonable amount in
the Vagrantfile, and update the docs to explain how to raise/lower it if
necessary (and recommend a minimum).

Vagrantfile

@@ -24,4 +24,9 @@ Vagrant.configure(VAGRANT_CONFIG_VERSION) do |config|
       salt.verbose = true
       salt.log_level = "info"
   end
+
+  config.vm.provider "virtualbox" do |vb|
+      vb.memory = "4096"
+      vb.cpus = "4"
+  end
 end