Update dependencies

[jayands]

We're on Jest 22 now, and though I doubt much functionality as far as this plugin is concerned has changed, it might be worth the minor version bump.

...Actually, reading the changelog, there's a whole bunch of changes and newness in jest-snapshots. It'd definitely be worth the bump, especially since version 22 plans to drop Node 4 (jestjs/jest#4769) support, so you can too.