Replace `tv4` JSON Schema validator

[plroebuck]

As the tv4 package no longer seems to get any love (last updated in Sep 2015), I've looked around for a replacement.

Benchmarks show the 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.