PDB-554 Switch to using new experimental ec2 beaker driver

This patch modifies our Gemfile and acceptance configuration so that we are
using my experimental EC2 backend driver from beaker.

At the moment its just pointing at my branch, so we can continue to test this
with our real Jenkins runs before releasing it to beaker proper.

Signed-off-by: Ken Barber <ken@bob.sh>
PDB-554 Remove temp refspec entry

Signed-off-by: Ken Barber <ken@bob.sh>
Merge pull request #909 from kbarber/ticket/1.5.x/pdb-554-cutover-to-new-experimental-ec2-beaker-driver

PDB-554 Switch to using new experimental ec2 beaker driver
Merge branch '1.5.x' into 1.6.x

* 1.5.x:
  PDB-554 Remove temp refspec entry
  PDB-554 Switch to using new experimental ec2 beaker driver