Update version to 17.3.0-rc1

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
diff --git a/VERSION b/VERSION
index 12c2e3a..dd84678 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-17.3.0-devel
+17.3.0-rc1