update_engine: bump minor version to 5

We recently disable puffin in minor version 4 and enabled it in minor version
5. This patch bumps the minor version to 5.

BUG=chromium:810184
TEST=brillo_update_payload {generate|verify}

Change-Id: I06b92f792fbc05c0acec062f8f3ff5d7305bd0a0
Reviewed-on: https://chromium-review.googlesource.com/917306
Commit-Ready: Amin Hassani <ahassani@chromium.org>
Tested-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Ben Chan <benchan@chromium.org>
diff --git a/update_engine.conf b/update_engine.conf
index e3f246f..3358411 100644
--- a/update_engine.conf
+++ b/update_engine.conf
@@ -1,2 +1,2 @@
 PAYLOAD_MAJOR_VERSION=2
-PAYLOAD_MINOR_VERSION=4
+PAYLOAD_MINOR_VERSION=5