DOC: Recommend sphinx 1.5 for now (pandas-dev#16929)

TomAugspurger · 3553x · commit abde16b1e9cc · 2017-07-16T16:03:00.000+01:00
For the SciPy sprint tomorrow, until the cause of the doc-building slowdown is fully identified.
diff --git a/ci/requirements_all.txt b/ci/requirements_all.txt
@@ -2,7 +2,7 @@ pytest
 pytest-cov
 pytest-xdist
 flake8
-sphinx
+sphinx=1.5*
 nbsphinx
 ipython
 python-dateutil
