Brew recipe recently switched from github tar ball to PyPI tar ball because of the introduction of submodules. The resulting tar ball appears to be missing some non-Python files.

.. WARNING: Invalid reStructuredText found in help - [[Errno 2] No such file or directory: '/home/john/.linuxbrew/Cellar/planemo/0.9.0/libexec/lib/python2.7/site-packages/planemo_ext/galaxy/util/docutils_template.txt'].