#221 PDFJS is not defined working fix, compatible with latest pdf.js release

[czapkowicz]

PDFJS name is completely removed in latest pdf.js release. More info here:
mozilla/pdf.js#9493

[sargin48]

155 // PDFJS.disableWorker = true;

and

    if (url && url.length) {
      // pdfLoaderTask = PDFJS.getDocument(params);
      pdfLoaderTask = pdfjsLib.getDocument(url);