Bump update_engine version to 8

This will enable zucchini diffing on all devices. Zucchini has been
tested on cuttlefish via asit/ota/zucchini_test for ~6 month. We are now
ready to ship this. Expected gain is 4%-5% OTA size reduction.

Test: th
Change-Id: Ic791fff3c15bcfe22a722b35367437c11f40efff
diff --git a/update_engine.conf b/update_engine.conf
index b6ca3c4..2d3a655 100644
--- a/update_engine.conf
+++ b/update_engine.conf
@@ -1,2 +1,2 @@
 PAYLOAD_MAJOR_VERSION=2
-PAYLOAD_MINOR_VERSION=7
+PAYLOAD_MINOR_VERSION=8