[rtd] require sphinx >= 1.5 for debugging purposes

requirements-rtd.txt

@@ -1,7 +1,7 @@
 # This file is sourced by readthedocs.org to install missing dependencies.
 # We need a more recent version of Sphinx for being able to provide
 # our own docutils.conf.
-Sphinx>=1.4.0
+Sphinx>=1.5.0
 cython
 numpy