pip: Fix Sphinx version temporarily
pip: Fix Sphinx version temporarily

Because RyuLaTeXBuilder(source/ryubuilder.py) supposes the old Sphinx,
Ryu-Book could not be built with Sphinx 1.5 or later.

This patch fixes Sphinx version temporarily.

Signed-off-by: IWASE Yusuke <bb80e84cc9f5a8f01385872ee89032beb5d4eba7@gmail.com>

osrg/ryu-book,iwaseyusuke/ryu-book,iwaseyusuke/ryu-book,osrg/ryu-book