psutil.cpu_times_percent() may produce negative results.

[giampaolo]

Just saw this failing on Travis / Linux:

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FAIL: test_sys_per_cpu_times_percent (__main__.TestSystemAPIs)
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Traceback (most recent call last):
  File "test/test_psutil.py", line 841, in test_sys_per_cpu_times_percent
    self._test_cpu_percent(percent)
  File "test/test_psutil.py", line 809, in _test_cpu_percent
    self.assertGreaterEqual(percent, 0.0)
AssertionError: -100.0 not greater than or equal to 0.0
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