erltrek_ship: notify commander when ship is destroyed.

* This will help make sure the commander doesn't try to
  call the ship after it has been killed (even though
  it is monitoring the ship process).


I got a whole bunch of these:

=ERROR REPORT==== 26-Mar-2014::21:13:28 ===
** State machine <0.117.0> terminating
** Last event in was timeout
** When State == idle
**      Data  == {state,<0.116.0>,1000}
** Reason for termination =
** {noproc,{gen_server,call,[<0.116.0>,count_nearby_enemies]}}
erltrek_ship: refactor update_condition.