Attempt to fix intermittent failing specs 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, by collecting the
expected failure time more precisely.

This other spec also fails occasionally:

  1) Feeding Dosing doses
     Failure/Error: dose.finished_at.to_f.should be_within(0.1).of(DateTime.now.to_f)
       expected 1374983237.304906 to be within 0.1 of 1374983237.8838968
     # ./spec/features/pump_control_spec.rb:54:in `block (3 levels) in <top (required)>'

In this case, just loosen the acceptable window for finished_at.

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.
Running these specs on a heavily-loaded CI server exacerbates the
fragility of these timing-based specs.

fixup! Attempt to fix intermittent failing spec for dosing exceptions
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