Updating requirement on Sphinx version

Our previous requirement was there because the mathjax extension didn't
work correctly with sphinx <= 1.5, and breathe required sphinx >= 1.4.
The latest breathe requires sphinx >= 1.8.4 though, and when trying
1.8.5 the problem with mathjax seems to be gone. Thus, I'm pinning the
sphinx version to what I know works.

This changes is for documentation only, so let's [skip ci].

Signed-off-by: Rodrigo Tobar <rtobar@icrar.org>

docs/requirements.txt

@@ -1,2 +1,2 @@
-Sphinx==1.4
+Sphinx==1.8.5
 breathe