Call update() when reparing signs.

It boggles the mind that this tiny little class has worked as intended
since 2011 (!!), and all of a sudden, signs no longer retain their text
in the repair procedure...

By calling the somewhat arbitrary `update()` method on the sign after
setting the contents, the sign appears to correctly update again.

Fixes #772

src/main/java/com/garbagemule/MobArena/repairable/RepairableSign.java

@@ -27,6 +27,7 @@ public class RepairableSign extends RepairableAttachable
         s.setLine(1, lines[1]);
         s.setLine(2, lines[2]);
         s.setLine(3, lines[3]);
+        s.update();
     }
 
 }