Add requirements.txt for old sphinx version used here

Our ReadTheDocs project config now specifies the use of a requirements.txt file
so builds for old branches started failing without it.

(cherry picked from commit 88a376822ef62192eca19030963b85301475446d)

requirements.txt

@@ -1,3 +1,7 @@
-# pinning sphinx to 1.4.* due to search issues with rtd:
-# https://github.com/rtfd/readthedocs-sphinx-ext/issues/25
-sphinx ==1.4.*
+# Base dependencies
+
+# Sync with readthedocs:
+# https://github.com/readthedocs/readthedocs.org/blob/master/requirements/pip.txt
+# https://github.com/readthedocs/readthedocs.org/blob/master/requirements/local-docs-build.txt
+sphinx==1.8.5  # pyup: <2.0.0
+sphinx_rtd_theme==0.4.3