remove pin for version of berkshelf
remove pin for version of berkshelf

Berkshelf 7 has gotten stable and includes a fix for handling tar
archives that have extended headers (that are appearing in cookbooks
these days).

This should fix our omnibus build erroring out while attempting to
vendor the cookbooks required by the internal cookbook.

Signed-off-by: Robb Kidd <11cd4f9ba93acb67080ee4dad8d03929f9e64bf2@chef.io>

chef/supermarket,robbkidd/supermarket,chef/supermarket,chef/supermarket,chef/supermarket,robbkidd/supermarket,robbkidd/supermarket,robbkidd/supermarket,chef/supermarket