Update version to 18.3.0-rc1

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
diff --git a/VERSION b/VERSION
index 8b16de0..bb28ad2 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-18.3.0-devel
+18.3.0-rc1