Travis error is MacOS image tolerance of 0.005 for `test_constrained_layout.py::test_colorbar_location`

_Originally posted by @jklymak in https://github.com/matplotlib/matplotlib/pull/12431/comment#issuecomment-427675481_

Travis error is MacOS image tolerance of 0.005 changing for `test_constrained_layout.py::test_colorbar_location`.  Not sure what changed on macOS to cause this, but it doesn't affect this PR.