graphics/gscan2pdf: update to 2.13.5
Now also supports ImageMagick7 (follows DEFAULT_VERSIONS)
Announcement:
https://sourceforge.net/p/gscan2pdf/mailman/message/59255852/
Never mind the typo in the announcement, its actually 2.13.5:
https://sourceforge.net/p/gscan2pdf/code/ci/v2.13.5/tree/