Update Doxyfile to a more recent version
Update Doxyfile to a more recent version

This was done with doxygen 1.8.8, Travis uses 1.8.7

bluerover/6lbr,arurke/contiki,bluerover/6lbr,arurke/contiki,MohamedSeliem/contiki,bluerover/6lbr,MohamedSeliem/contiki,MohamedSeliem/contiki,bluerover/6lbr,bluerover/6lbr,arurke/contiki,arurke/contiki,arurke/contiki,arurke/contiki,bluerover/6lbr,arurke/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,bluerover/6lbr,MohamedSeliem/contiki,MohamedSeliem/contiki