bump version to 1.13.0-rc7

Signed-off-by: Victor Vieux <vieux@docker.com>
diff --git a/VERSION b/VERSION
index b6f4544..b849e03 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-1.13.0-rc6
+1.13.0-rc7