Switch to gp2 disks

They cost twice as much, but in our usage scenario this means $5/month
instead of $2.50/month and they shave several minutes off of sandbox
builds (and will probably aid other builds).

The continuous_delivery launch_instance playbook is already using gp2

playbooks/roles/launch_ec2/tasks/main.yml

@@ -50,6 +50,7 @@
       - device_name: /dev/sda1
         volume_size: "{{ root_ebs_size }}"
         delete_on_termination: true
+        volume_type: "gp2"
     zone: "{{ zone }}"
     instance_profile_name: "{{ instance_profile_name }}"
     user_data: "{{ user_data }}"