Pin sphinx to 1.4 when generating docs to workaround search issues on RTD

Works around https://github.com/rtfd/readthedocs.org/issues/2708 and fixes #360.
Patch from @nicoddemus.

requirements.txt

@@ -0,0 +1,3 @@
+# pinning sphinx to 1.4.* due to search issues with rtd:
+# https://github.com/rtfd/readthedocs-sphinx-ext/issues/25
+sphinx ==1.4.*