Need to ignore $ref URN

[almirus]

if(schema.$ref && typeof schema.$ref !== "object" && schema.$ref.substr(0,1) !== "#" && schema.$ref.substr(0,3) !== "urn" && !this.refs[schema.$ref]) { refs[schema.$ref] = true; }

[Tobias Munk]

This repo is no longer maintained (see also #800)

Development is continued at https://github.com/json-editor/json-editor

For details please visit json-editor/json-editor#5