Vagrantfile improvements

- provision installs latest version of Docker, supports multi-line Dockerfiles
- provision builds the image
- use 2 virtual CPUs

Vagrantfile

@@ -5,13 +5,17 @@ $provision_script = <<SCRIPT
 set -ex
 sudo apt-get update && sudo apt-get install -y docker.io
 sudo gpasswd -a vagrant docker
+curl -L https://get.docker.com/ | sh
+docker build -t phantomjs2 - < /vagrant/Dockerfile
+echo "Docker image built."
 SCRIPT
 
 Vagrant.configure(2) do |config|
   config.vm.box = "ubuntu/trusty64"
 
   config.vm.provider "virtualbox" do |vb|
     vb.memory = "8192"
+    vb.cpus = 2
   end
 
   config.vm.provision "shell", inline: $provision_script