Bump version to 1.10.2-rc1

Signed-off-by: Tibor Vass <tibor@docker.com>
diff --git a/VERSION b/VERSION
index 5ad2491..9f0ff89 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-1.10.2
+1.10.2-rc1