Add key information to instance location when validating required fields of a schema

[klundberg]

This adds the currently validating key to the instanceLocation JSONPointer when validating required fields in a schema. We need this since we want to find out which fields specifically are missing from the data we're looking to upload, so that we can show errors in the right place in our UI that corresponds with each required field.

This also fixes a deprecation warning when using swift 5.0 and above, and fixes a compiler issue where the library was using iOS 11 api without declaring iOS 11 as a minimum deployment target.