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.

lib/spack/docs/conf.py

@@ -49,7 +49,6 @@
 # If extensions (or modules to document with autodoc) are in another directory,
 # add these directories to sys.path here. If the directory is relative to the
 # documentation root, use os.path.abspath to make it absolute, like shown here.
-sys.path.insert(0, os.path.abspath('exts'))
 sys.path.insert(0, os.path.abspath('../external'))
 if sys.version_info[0] < 3:
     sys.path.insert(0, os.path.abspath('../external/yaml/lib'))