Update perseus exercise plugin to version compatible with changes to core API.
Update perseus exercise plugin to version compatible with changes to core API.

mrpau/kolibri,aronasorman/kolibri,MingDai/kolibri,learningequality/kolibri,learningequality/kolibri,jonboiser/kolibri,DXCanas/kolibri,MingDai/kolibri,DXCanas/kolibri,lyw07/kolibri,MingDai/kolibri,benjaoming/kolibri,rtibbles/kolibri,indirectlylit/kolibri,aronasorman/kolibri,indirectlylit/kolibri,aronasorman/kolibri,rtibbles/kolibri,lyw07/kolibri,learningequality/kolibri,DXCanas/kolibri,DXCanas/kolibri,jonboiser/kolibri,indirectlylit/kolibri,lyw07/kolibri,indirectlylit/kolibri,lyw07/kolibri,jonboiser/kolibri,christianmemije/kolibri,benjaoming/kolibri,MingDai/kolibri,benjaoming/kolibri,rtibbles/kolibri,learningequality/kolibri,benjaoming/kolibri,christianmemije/kolibri,christianmemije/kolibri,rtibbles/kolibri,jonboiser/kolibri,mrpau/kolibri,mrpau/kolibri,christianmemije/kolibri,mrpau/kolibri,aronasorman/kolibri