Update pdfjs 0.8.1136

TODO:
 - [x] CSS isn't loaded (resolves to …/index.php/apps/files_pdfviewer/3rdparty/pdfjs/viewer.css, index.php shouldn't be there)
 - [x] pdf.worker.js isn't loaded (resolves to …/apps/build/worker.js)

Fixes #1265

cc @Mikka20 @PVince81