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

========

https://hg.mozilla.org/integration/gaia-central/rev/91fe480f4545
Author: Adam Hyland <protonk@gmail.com>
Desc: Bug 909343 [Clock] Refactor Date Math Utilities r=jugglinmike

Refactor utils.js to provide methods for:

- Conversion to milliseconds from objects with duration values, e.g.
  {hours: 4, seconds: 12} (for any combination of hours, minutes,
  seconds, etc.
- Conversion from millisecond interval to object with duration values.

b2g/config/gaia.json

@@ -1,4 +1,4 @@
 {
-    "revision": "3057b7d5e2cce7048190daab170e8b8a11cca1bf", 
+    "revision": "91fe480f454592d5ae21d29d2291966ffbd52da2", 
     "repo_path": "/integration/gaia-central"
 }