Switch supported minor version to 3.

Bug: 23182225
TEST=cros_workon_make update_engine --test

Change-Id: Ide798f89bd0e3f662d2deac96035780892bbfa08
diff --git a/update_engine.conf b/update_engine.conf
index 981887d..449e669 100644
--- a/update_engine.conf
+++ b/update_engine.conf
@@ -1,2 +1,2 @@
 PAYLOAD_MAJOR_VERSION=2
-PAYLOAD_MINOR_VERSION=2
+PAYLOAD_MINOR_VERSION=3