Attempt to fix intermittent failing spec for dosing exceptions

This spec occasionally fails:

  1) Feeding Dosing records exceptions as failures
     Failure/Error: dose.finished_at.to_f.should be_within(0.2).of(DateTime.now.to_f)
       expected 1374982365.680259 to be within 0.2 of 1374982365.9335904
     # ./spec/features/pump_control_spec.rb:116:in `block (3 levels) in  <top (required)>'

Try to address the problem, at least partially, but loosening the
acceptable time window.  This functionality is just plain hard to test
because it runs in a different thread and the main application code
doesn't join this thread.