Updates to JSON Schema Validators section

[chris48s]

I came here to add v8r to this section, but while I was here I noticed a couple of other things.

1. AJV's description was quite out of date with reference to the specification versions it has compatibility with
2. request-validator is deprecated (see the README). I'd suggest this make it not-awesome. The author now points users towards https://github.com/bugventure/jsen which is already listed elsewhere in this section. I suggest removing this