Done with a temporary solution to get our Linux build to work on Travis-CI.

We are using ppa:beineri/opt-qt531, but that PPA misses
/opt/qt53/plugins/printsupport/libcupsprintersupport.so, so we are
getting off the OpenCOR website. This is clearly a temporary solution
until, hopefully, the PPA author makes
/opt/qt53/plugins/printsupport/libcupsprintersupport.so available in
his PPA…