Ranker throws exception when compilation fails

[Procrat]

Like #90, not fatal for the ranker, but fatal for that bot, as was the case with @javache's.

Traceback (most recent call last):
  File "/home/battlebots/production/battlebots/ranker/ranker.py", line 127, in battle_loop
    battle(loop)
  File "/home/battlebots/production/battlebots/ranker/ranker.py", line 71, in battle
    compilation_success = all(bot.compile() for bot in bots)
  File "/home/battlebots/production/battlebots/ranker/ranker.py", line 71, in <genexpr>
    compilation_success = all(bot.compile() for bot in bots)
  File "/home/battlebots/production/battlebots/database/models.py", line 136, in compile
    timeout=timeout, stdout=sp.PIPE, stderr=sp.PIPE)
  File "/home/battlebots/production/battlebots/backports.py", line 51, in sp_run
    stderr=stderr)
TypeError: __init__() got an unexpected keyword argument 'stderr'