Update version for v2.4.0-rc1 release

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
diff --git a/VERSION b/VERSION
index 6c91a81..6b6b254 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-2.3.90
+2.3.91