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

========

https://hg.mozilla.org/integration/gaia-central/rev/975e8d596def
Author: Julien Wajsberg <felash@gmail.com>
Desc: Bug 891811 - [sms] tests: properly use sinon so that we have no side effects between suites r=gnarf

properly use sinon sandboxes for good restore

b2g/config/gaia.json

@@ -1,4 +1,4 @@
 {
-    "revision": "966752354a417850577f86c9246bf46ed02e7774", 
+    "revision": "975e8d596def8934f04193e5955371a66e541d8e", 
     "repo_path": "/integration/gaia-central"
 }