ruby: drop the [] notation for lookup function.

ruby: drop the [] notation for lookup function.

This is in preparation for adding a second arugment to the lookup
function for the CacheMemory class.  The change to *.sm files was made using
the following sed command:

sed -i 's/\[\([0-9A-Za-z._()]*\)\]/.lookup(\1)/' src/mem/protocol/*.sm