Upgrade rust/crates/regex to 1.3.9

Test: None
Change-Id: Ic5726cda9566f808b7dd21d8ce9a3045250f8a54
diff --git a/METADATA b/METADATA
index 53b95fe..6eff2f3 100644
--- a/METADATA
+++ b/METADATA
@@ -9,11 +9,11 @@
     type: GIT
     value: "https://github.com/rust-lang/regex"
   }
-  version: "1.3.7"
+  version: "1.3.9"
   license_type: NOTICE
   last_upgrade_date {
     year: 2020
-    month: 4
-    day: 17
+    month: 5
+    day: 28
   }
 }