Update snakemake version

[dfm]

The most recent version of snakemake is 7.20.0, but we're currently pinned to 7.15.2. As far as I can tell, the only thing that breaks is the cache optimization discussed in #124. In the short term it seems like it should be possible to update this code (if I can ever dig in properly to understand what's going on), but it might be better to take a stab at reworking the caching infrastructure in the long term.

This should also fix #258