Tweak Vagrant config for improved performance

Summary: Ran into some issues with Vagrant hitting the memory limit of 8GB.  Bumping to 12GB appears to have mitigated the issue.  Also disabled the GUI because it adds an additional performance constraint.

Test Plan: Tested locally and it appears to improve performance on my machine.  I have not run into issues of Vagrant stalling since I've made this change.

Reviewers: deadalnix, schancel, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1641

contrib/gitian/Vagrantfile

@@ -12,11 +12,12 @@ Vagrant.configure("2") do |config|
   config.vm.synced_folder '../..', '/vagrant'
 
   config.vm.provider "virtualbox" do |vb|
-    # Display the VirtualBox GUI when booting the machine
-    vb.gui = true
+    # Uncomment to display the VirtualBox GUI when booting the machine
+    # This may be useful for debugging.
+    #vb.gui = true
 
     # Customize the amount of memory on the VM:
-    vb.memory = "8048"
+    vb.memory = "12288"
     vb.cpus = 4
   end