CLeaned up Rakefile and kitchen.yml

salt-shaker is now available as a separate Rake task so you can
use rake to run salt-shaker. pre_create command has been removed
from kitchen config
Overlooked run_shaker function call.

run_shaker is no longer available as a function. It has been replaced as
a dependent rake task.
Merge pull request #8 from ministryofjustice/cleanup

Cleaned up Rakefile and kitchen.yml