lock the sphinx command to version 1.5.6, since version 1.6.x seems to break...

lock the sphinx command to version 1.5.6, since version 1.6.x seems to break one of the extensions we use

doc/Makefile

@@ -158,7 +158,7 @@ $(VENV):
 	@( \
 		virtualenv -p $(PYTHON) $(VENV); \
 		. $(VENV)/bin/activate; \
-		pip install Sphinx; \
+		pip install Sphinx==1.5.6; \
 		pip install sphinxcontrib-images; \
 		deactivate;\
 	)