Fix vibrating square

The Gehennom changes broke the vibrating square, allowing hero to go
down into the Sanctum via stairs without performing the invocation.

Fix this by making the hellfill lua check for invocation level, and
placing down the vibrating square trap, instead of stairs.

dat/hellfill.lua

@@ -257,6 +257,10 @@ hells[hellno]();
 --
 
 des.stair("up")
-des.stair("down")
+if (u.invocation_level) then
+   des.trap("vibrating square");
+else
+   des.stair("down")
+end
 
 populatemaze();