Fix METADATA version

Steven updated this to 2.6.4 last year.

Test: treehugger
Change-Id: I38f4a692ed990106259f2a98052e3408149271c4
diff --git a/METADATA b/METADATA
index c6e6456..641829e 100644
--- a/METADATA
+++ b/METADATA
@@ -11,7 +11,7 @@
     type: GIT
     value: "https://github.com/westes/flex"
   }
-  version: "flex-2.5.39"
-  last_upgrade_date { year: 2018 month: 5 day: 22 }
+  version: "flex-2.6.4"
+  last_upgrade_date { year: 2018 month: 5 day: 29 }
   license_type: NOTICE
 }