jobserver: Jobserver doesn't work on Python3.4

[jbohren]

I noticed it when running the tests for #196, but it appears that it's even an issue in the current branch. However, people probably aren't noticing because in the current version warnings are suppressed unless the package fails.

See here: https://travis-ci.org/catkin/catkin_tools/jobs/61523929#L579

[jbohren]

I believe this is the culprit: https://docs.python.org/3.4/library/os.html#fd-inheritance

[jbohren]

This is resolved by the combination of #196 and osrf/osrf_pycommon#10

[jbohren]

Jobserver works on 3.4 with changes in #249.