Use the sharper PDF version of the LiU logo

The PDF version is a vector graphics version of the logotype, unlike
the PNG version (which is a raster version). This gives a sharper
result (mainly visible when printing), but also slightly reduces
the file size of the produced report PDF file.

The logotype has been downloaded from:
https://liu.se/insidan/kommunikationsstod/grafiskprofil



Signed-off-by: Martin Erola <marer248@student.liu.se>