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

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https://hg.mozilla.org/integration/gaia-central/rev/891d459c0d6b
Author: Sam Joch <samuel@guiora.com>
Desc: Merge pull request #16034 from samjoch/bug-968780-unit-tests-in-b2g-on-mac

Bug 968780 - unit tests in b2g before script failed on mac r=julienw

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https://hg.mozilla.org/integration/gaia-central/rev/6ba2330cf088
Author: Sam Joch <samuel@guiora.com>
Desc: Bug 968780 - unit tests in b2g before script failed on mac r=julienw

b2g/config/gaia.json

@@ -1,4 +1,4 @@
 {
-    "revision": "e92e4f29a2e9f1323bcc0858e471b0837ac07135", 
+    "revision": "891d459c0d6be67094b20eca57c98289138254db", 
     "repo_path": "/integration/gaia-central"
 }