Replace `tv4` JSON Schema validator

As the [tv4](https://github.com/geraintluff/tv4) package no longer seems to get any love (last updated in Sep 2015), I've looked around for a replacement.

[Benchmarks](https://github.com/ebdrup/json-schema-benchmark/) show the [ajv](https://github.com/epoberezkin/ajv) package to be an excellent candidate -- it's about 50 times faster than `tv4` and passes all tests (except one which no JS implementation can). It also supports
JSON Schema draft-07. While not a drop-in replacement, its API is very similar in many respects to `tv4` so _hopefully_ minimal changes will be needed for the migration.