gzip labslink files prior to upload

Fixes file size issue reported by Europe PMC:

> Dear Dryad Digital Repository,
>  
> The links file that you update every Sunday: "labslink-links.xml" (for the Europe PMC External Links Service), has just gone over the 2MB-per-file limit, and therefore cannot be processed. You need to either start compressing the file before uploading, or begin writing to a second links file.
> 
> Files must be no bigger than 2 MB each. The total file size limit is 500 MB. All files must be named with one of the following extensions:
>  
> .xml
> .zip
> .gz
> .tar
> .tar.gz
> .targz