Add enrage script to creature 1511 (Enrage Silverback Gorilla)

Related to https://github.com/classicdb/database/issues/629

Sources: https://www.youtube.com/watch?v=cO671tvyeiQ
http://www.wowhead.com/npc=1511#abilities

Signed-off-by: Cala <calaftp@free.fr>