Make compatible with ownCloud 8

Some bloody hacks to make files_pdfviewer with ownCloud 8 working again and bump the PDFJS version.

Fixes https://github.com/owncloud/files_pdfviewer/issues/26, https://github.com/owncloud/files_pdfviewer/issues/27, https://github.com/owncloud/files_pdfviewer/issues/25, https://github.com/owncloud/files_pdfviewer/issues/24 and https://github.com/owncloud/files_pdfviewer/issues/20