Remove pinned chef dependency

Previously we were pinned against chef 10.12.0 due to conflicts with the bunny gem.

Since chef 11 is now in use, and this is the default that we are running, we can remove that dependency.

This will resolve any problems we would have trying to install an older version of chef which depends on net-ssh ~> 2.2.2 which doesn't work too well.

@wjlroe  - Thoughts?