Regex failing for checksums fix.

Setting a range for the checksum so it works with sha256 checksum length. This fixes a bug where my setting the checksum to the default checksum on one node to ensure it got the default version resulted in a chef run fail reporting that the checksum didn't match the regex.

If this was not the intended use please let me know a better way...

Ticket: https://tickets.opscode.com/browse/COOK-3569