Deterministically compress files

Gzip contains a timestamp in the compressed stream. If you don't explicitly give it one then the Python module will include the current time instead. This means that if you just blindly compress everything anytime a file changes and try to commit those artifacts then you end up with a new .gz file everytime. This change will make it so that the output is the same as long as the files are the same (including modification time).

If you accept this I'd love a new release with it as well :)