Fix "cookbook drush not found" error

Either the name of the repository changed from `drush` to `chef-drush`
at some point in the past, or everyone's been cloning to a folder named
`drush`...

Either way, we should refer to the cookbook by it's correct machine
name: `chef-drush`.
Merge branch 'feature-fix_cookbook_not_found_error' into master-malldatabase