Use --rsyncable for gzip archives

[yglodt]

According to the gzip manpage, using the --rsyncable flag improves data usage:

--rsyncable
While compressing, synchronize the output occasionally based on the input.
This increases size by less than 1 percent most cases,  but  means  that  the 
rsync(1)  program can take advantage of similarities in the uncompressed input when 
syncronizing two files compressed with this flag. gunzip cannot tell the difference
between a compressed file created with this option, and one created without it.