contint: djvulibre-bin for mw djvu unit tests

Thomas (Tpt) is writing tests for MediaWiki djvu support [0]. We thus
need the djvu utilities installed on contint slaves.

[0] https://gerrit.wikimedia.org/r/#/c/98258/

Change-Id: Ic932c4ababfc9b66c4de79d3251633618b823ebe