Use new quality-of-life improvements from vcr-cassette

[mpalmer]

This exposes the ability to provide "matchers" for request bodies, and specify your JSON response bodies as JSON, new features recently added to vcr-cassette.

(Merge note: this PR depends on http-rs/vcr-cassette#22; the patch.crates-io reflects that, for CI purposes. I will force-push a version without that change to Cargo.toml before merge)