upgrade s3fs to support objects larger than 5GB

Once [this s3fs fix](https://github.com/dask/s3fs/commit/05fc3d9c64a5f9ff701f9d9541f707aff5750349) is released, we can update Triage's s3fs requirement, remove the `S3Store` open/write shim, and test Triage against it – as indicated by #687.

That PR's unit test should be preserved, and should pass, as a result of the update. It wouldn't be a bad idea to then run the PR's functional test, as well, to confirm.