Bumping gaia.json for 1 gaia-central revision(s) a=gaia-bump

========

https://hg.mozilla.org/integration/gaia-central/rev/df31e3d14be6
Author: Zac Campbell <zcampbell@mozilla.com>
Desc: Revert "Bug 921721 - hotfix to bust JS Exception in startup a=testonly r=me"

This reverts commit 5cf27888e390526c072d4b44a5af54c8aaf7205a.

b2g/config/gaia.json

@@ -1,4 +1,4 @@
 {
-    "revision": "17d3c31fb43696cda00ae13b79b8f2053fa12e11", 
+    "revision": "df31e3d14be6612c530d0bc8d4e28b865c7daaf3", 
     "repo_path": "/integration/gaia-central"
 }