Bump guiunit to get updated min macOS version.

Commit list for mono/guiunit:

* mono/guiunit@9f67042 Bump min macOS version to 10.9.
* mono/guiunit@0c3159a [Harness] Fix exit code of 0 being reported in case of exceptions in the harness
* mono/guiunit@9b7497c Merge pull request #15 from mono/fix-more-warnings
* mono/guiunit@a264470 Fix more warnings
* mono/guiunit@dd094e7 Merge pull request #14 from mono/fix-warnings
* mono/guiunit@b3afede Fix build warnings

Diff: mono/guiunit@1306b0d...9f67042