Don't upgrade core

Signed-off-by: Raphaël Barrois <raphael.barrois@polytechnique.org>

diff --git a/core b/core
index d3e8ac4..df1ec68 160000 (submodule)
--- a/core
+++ b/core
@@ -1 +1 @@
-Subproject commit d3e8ac41247fb278285704bd8bf819a5275e5aca
+Subproject commit df1ec683f930734d7532bd3dc3b10c46da947748