update_payload: uprev to minor version 6

BUG=chromium:920699
TEST=unittest
TEST=check_update_payload --check # with a delta payload

Change-Id: I40bc1e535ed241b74c6d889757a8dcf046bac0de
diff --git a/scripts/update_payload/checker.py b/scripts/update_payload/checker.py
index 15f11ae..6d17fbe 100644
--- a/scripts/update_payload/checker.py
+++ b/scripts/update_payload/checker.py
@@ -71,6 +71,7 @@
     3: (_TYPE_DELTA,),
     4: (_TYPE_DELTA,),
     5: (_TYPE_DELTA,),
+    6: (_TYPE_DELTA,),
 }
 
 _OLD_DELTA_USABLE_PART_SIZE = 2 * 1024 * 1024 * 1024