handle update to the MissingRegError constructor

[chevett]

Hello, I am trying to run the latest code and I am seeing

1) async schema loading
       $merge
         should load missing schemas:
     TypeError: resolver.resolve is not a function
      at resolveUrl (node_modules/ajv/dist/compile/resolve.js:90:21)
      at new MissingRefError (node_modules/ajv/dist/compile/ref_error.js:7:52)
      ...

  2) async schema loading
       $patch
         should load missing schemas:
     TypeError: resolver.resolve is not a function
      at resolveUrl (node_modules/ajv/dist/compile/resolve.js:90:21)
      at new MissingRefError (node_modules/ajv/dist/compile/ref_error.js:7:52)
      at getSchema (keywords/add_keyword.js:2:1397)
      ...

Looks like https://github.com/ajv-validator/ajv/pull/1862 changed the signature of MissingRefError.