Pin Sphinx to <1.2

1.2 was just released and has a bug that causes builds to fail with
the error:

  error: 'source_dir' must be a directory name (got `...doc/source`)

See: http://bugs.python.org/issue19570
and: https://bitbucket.org/birkenfeld/sphinx/pull-request/193/builddoc-shouldnt-fail-on-unicode-paths/diff

Change-Id: I8a27127dd298f0c8a905c4ab17b3112e010059fd