bump version to 17.06.1-ce-rc4

Signed-off-by: Andrew Hsu <andrewhsu@docker.com>
diff --git a/VERSION b/VERSION
index 43cd08c..97b8d7a 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-17.06.1-ce-rc3
+17.06.1-ce-rc4