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`)

Change-Id: Iec882c2b0cf96d20828b57005fb336be2593ad6b
See: http://bugs.python.org/issue19570
and: https://bitbucket.org/birkenfeld/sphinx/pull-request/193/builddoc-shouldnt-fail-on-unicode-paths/diff