List Debian Stretch as supported

ekohl · ekohl · commit 6e0ee20c78fb · 2017-07-26T18:30:15.000+02:00
db51029 added support for Stretch but didn't add this to the metadata.
diff --git a/metadata.json b/metadata.json
@@ -56,7 +56,8 @@
       "operatingsystemrelease": [
         "6",
         "7",
-        "8"
+        "8",
+        "9"
       ]
     },
     {
