* Sphinx no longer supports Python 2.6
* Update vendored sphinxcontrib.programoutput from 0.9.0 to 0.10.0
* Documentation cannot be built in parallel
* Let Travis install programoutput for us
* Remove vendored sphinxcontrib-programoutput
Recent updates to the sphinx package prevent the vendored version
from being found in sys.path. We don't vendor sphinx, so it doesn't
make sense to vendor sphinxcontrib-programoutput either.
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