Bumping gaia.json for 2 gaia revision(s) a=gaia-bump

========

https://hg.mozilla.org/integration/gaia-central/rev/500d57309898
Author: Etienne Segonzac <etienne@segonzac.info>
Desc: Merge pull request #15990 from etiennesegonzac/contact-thumbnails

Bug 967272 - Introducing contact photo thumbnails for new contacts,
and using thumbnails wherever it makes sense.
r=arcturus, jmcf(facebook), julienw(sms), rik(dialer)

========

https://hg.mozilla.org/integration/gaia-central/rev/6c4783a162d4
Author: Etienne Segonzac <etienne@segonzac.info>
Desc: Bug 967272 - Introducing contact photo thumbnails for new contacts,
and using thumbnails wherever it makes sense.
r=arcturus, jmcf(facebook), julienw(sms), rik(dialer)

b2g/config/gaia.json

@@ -1,4 +1,4 @@
 {
-    "revision": "042ee12cec91ebf1eaa20f926da1c5a1b7087cdd", 
+    "revision": "500d57309898b0514b1ad1a61a8d8274909428a4", 
     "repo_path": "/integration/gaia-central"
 }