[FAB-18183] Bump sphinx in requirements.txt to v1.8.5

This patch bumps sphinx to v1.8.5 to use the same version as Pipfile.

Signed-off-by: Tatsuya Sato <Tatsuya.Sato@hal.hitachi.com>
(cherry picked from commit 57f2834)

docs/requirements.txt

@@ -11,6 +11,6 @@ Pygments==2.1.3
 pytz==2016.4
 six==1.10.0
 snowballstemmer==1.2.1
-Sphinx==1.7.2
+Sphinx==1.8.5
 sphinx-rtd-theme==0.2.5b2
 recommonmark==0.4.0