Ha! Add CPUs to VM

In order to run concurrency code that relies on there being multiple cores, we acutally need this VM to *have* multiple cores...

In doing this I also found some nicer syntax for the memory, so this commit also amends that.

Vagrantfile

@@ -9,7 +9,8 @@ Vagrant.configure("2") do |config|
 
   # Need more memory for large arrays
   config.vm.provider :virtualbox do |v|
-    v.customize ["modifyvm", :id, "--memory", 2048]
+    v.memory = 2048
+    v.cpus = 4
   end
 
   config.vm.provision "shell", inline: <<-SHELL