Docs: Fix breakage with Sphinx 1.5 and upper

Commit 85c21e5c (docs-rst: better adjust margins and font size) added a
\usepackage{geometry} that conflicts with another inclusion deep within the
dependencies with newer versions of Sphinx, causing the the PDF build to fail
with a "conflicting parameters" error.

Detect the Sphinx version, using sphinxsetup for Sphinx versions 1.5 and
upper.

Fixes: 85c21e5c
[jc: Tweaked logic to exclude 1.5.x for x < 3 ]
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>