Remove gzip option, always set this to true

[mauritsvanrees]

From the readme:

Use repozo's zipping functionality. true by default. Set it to false and repozo will not gzip its files. Note that gzipped databases are called *.fsz, not *.fs.gz. Changed in 0.8: the default used to be false, but it so totally makes sense to gzip your backups that we changed the default.

Now that I write this, I remember that there is a gzip option in ZODB as well, so you store zipped versions of transactions, or something like that. This should mean that zipping the Data.fs backup would make very little difference. So perhaps we should keep the option.
If there is a lot of code that can be dropped, it could still be nice.