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

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https://hg.mozilla.org/integration/gaia-central/rev/b919f497d334
Author: Sam Foster <sam@sam-i-am.com>
Desc: Merge pull request #20111 from sfoster/bug-1020435-strings

Bug 1020435 - 'Hold an' not 'Hold on an' icon in new FTU strings. r=fcampo

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https://hg.mozilla.org/integration/gaia-central/rev/c4873736ae00
Author: Sam Foster <sfoster@mozilla.com>
Desc: Bug 1020435 - One word fix to finalize FTU tutorial strings

b2g/config/gaia.json

@@ -4,6 +4,6 @@
         "remote": "", 
         "branch": ""
     }, 
-    "revision": "f07e8b6a3df5c70280d647ef69e8cf1c70018fb3", 
+    "revision": "b919f497d3345600c87a2e9a481933f27b6df45a", 
     "repo_path": "/integration/gaia-central"
 }