Bump the minor version

Test: unit tests pass, generate & apply a partial update
Change-Id: I05f8466c4a92cb3774030451d83005474d8fdc9d
diff --git a/update_engine.conf b/update_engine.conf
index af213ad..b6ca3c4 100644
--- a/update_engine.conf
+++ b/update_engine.conf
@@ -1,2 +1,2 @@
 PAYLOAD_MAJOR_VERSION=2
-PAYLOAD_MINOR_VERSION=6
+PAYLOAD_MINOR_VERSION=7