Bug 1460463 - Update .arcconfig to make history mutable; r=gps

Differential Revision: https://phabricator.services.mozilla.com/D1255

--HG--
extra : amend_source : bf7ad163b38455fe4ffe4ac38cc28e331cddabbe

.arcconfig

@@ -1,4 +1,5 @@
 {
   "phabricator.uri" : "https://phabricator.services.mozilla.com/",
-  "repository.callsign": "MOZILLACENTRAL"
+  "repository.callsign": "MOZILLACENTRAL",
+  "history.immutable": false
 }