Unpin the version of beaker

We had pinned beaker previously because we were waiting for some of our new EC2
customisations to be merged in and released. This has been done now.

Signed-off-by: Ken Barber <ken@bob.sh>
Merge pull request #965 from kbarber/maint/1.6.x/unpin-beaker

Unpin the version of beaker