Upgrade json schema to latest version

We've been using 1.1.1 for a long time, and 2.2.5 brings some fancy new features like strict mode. With this upgrade, we'll be able to depend the newer json-schema version in apps that use interpol.
We should probably make the version be an attribute of the endpoint. As this currently exists it looks like there's no way to make an endpoint use v4 json schema.

That said, I'm fine with that being declared out of scope if it'll be addressed in a future iteration.