Use tar.xz instead of zip format during update

[kleuter]

On recent macOS (10.10+) it's better to use modern and very efficient compression XZ format (based on LZMA2) which is approx. 33% smaller (most recent example: 14.5 MB for tar.gz against 21.7MB for ZIP)

This nice improvement will be actually used when v.1.5.10 will update itself to some future version of the app.