Remove metadata from PDF to avoid deploy updates when there is no change

[NorwegianKiwi]

Inside the PDF, created date and modified date are inserted when they are created. Remove this after creating the PDF.

One way that seems to work, is to run the following commands:

exiftool -all:all= original.pdf -o temp.pdf
qpdf --deterministic-id temp.pdf out.pdf

At least, this should be done on travis (since travis builds and deploys).
Need to find out how we can install exiftool and qpdf on travis, if possible.
Or, if there are other ways of making the metadata of two PDFs the same (given that they have the same content).