Sphinx no longer supports Python 2.6 (#4266)

* 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.

.travis.yml

@@ -86,7 +86,8 @@ install:
   - pip install --upgrade setuptools
   - pip install --upgrade codecov
   - pip install --upgrade flake8
-  - pip install --upgrade sphinx
+  - if [[ "$TEST_SUITE" == "doc" ]]; then pip install --upgrade sphinx; fi
+  - if [[ "$TEST_SUITE" == "doc" ]]; then pip install --upgrade sphinxcontrib-programoutput; fi
 
 before_script:
   # Need this for the git tests to succeed.