Fix activity.rerun not working on retried tasks

[jbbarth]

If task1 fails once, and is retried, you get two activities in the history: one scheduled at event ID "153" for instance, and the retry scheduled at event ID "159". Running simpleflow activity.rerun on the first task doesn't work, you'll get something like:

2017-09-01T14:30:24 INFO [process=MainProcess, pid=10692]: Found execution: workflowId=analysis_131427 runId=22yTLsUqa8MmcCYBUpoRdG3wKHr4lGgJ7Mal/e/FotamU=
Traceback (most recent call last):
  File "/usr/local/bin/simpleflow", line 11, in <module>
    sys.exit(cli())
  File "/usr/local/lib/python2.7/dist-packages/click/core.py", line 716, in __call__
    return self.main(*args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/click/core.py", line 696, in main
    rv = self.invoke(ctx)
  File "/usr/local/lib/python2.7/dist-packages/click/core.py", line 1060, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/local/lib/python2.7/dist-packages/click/core.py", line 889, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/usr/local/lib/python2.7/dist-packages/click/core.py", line 534, in invoke
    return callback(*args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/simpleflow/command.py", line 655, in activity_rerun
    history, scheduled_id=scheduled_id, activity_id=activity_id, input=input_override,
  File "/usr/local/lib/python2.7/dist-packages/simpleflow/swf/helpers.py", line 117, in find_activity
    raise ValueError("Couldn't find activity.")
ValueError: Couldn't find activity.

This is because simpleflow (or swf?) history removes the retries and only leaves the last activity for a given activity ID. This is a rough guess based on 3 mins of digging in the code, the issue might be a bit more subtle.

To be fixed next week!