message: Check version only when a input spec has  property
diff: @@ -95,6 +95,7 @@ function embed(el, spec, opt, callback) {
  spec = PREPROCESSOR[mode](spec);
  if (mode === MODES['vega-lite']) {
  + if (spec.$schema) {
  const parsed = url_parser(spec.$schema);
  var parsed_version = parsed.version.replace(/^v/g,'');
  if (versionCompare(parsed_version, VERSION['vega']) !== 0 ){
  @@ -102,6 +103,7 @@ function embed(el, spec, opt, callback) {
  + "but current version of \"vega\" is " + VERSION['vega'] + ".");
  }
  }
  + }
  // ensure container div has class 'vega-embed'
  var div = d3.select(el)
  