Diff-quality should not raise an exception due to pylint internal warnings.

Pylint may pipe output to stderr but still complete and exit
with a exit code of 0. The pylint report is still valid and
consumable by diff-quality. (A good example of this is a deprecation
warning due to pylintrc configuration.)
Merge pull request #116 from benpatterson/benp/pylint-handle-warnings

Diff-quality should not fail on pylint warnings.