bump version to 17.07.0-ce-rc2

Signed-off-by: Andrew Hsu <andrewhsu@docker.com>
diff --git a/VERSION b/VERSION
index b90ffa1..e83e8b0 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-17.07.0-ce-rc1
+17.07.0-ce-rc2