Updating Berks to 2.0.18

This is to resolve the broken berks config with the supermarket push.
This also fixes the strainerfile for the issue with the location of the
gems and running bundle exec.
Updated chefspec and rspec, and the spec_helper.

Change-Id: I21db7b77aa07bc7a7a0d47da135052617399b116
Merge "Updating Berks to 2.0.18" into stable/havana