Bumping gaia.json for 2 gaia-1_2 revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-1_2/rev/4a7e84f546b6
Author: Michael Henretty <michael.henretty@gmail.com>
Desc: Merge pull request #12839 from mikehenrty/bug-924251-overlay

Bug 924251 - fixing markup to use building blocks for offline overlay, r=alive

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https://hg.mozilla.org/integration/gaia-1_2/rev/37548bae0ebb
Author: Michael Henretty <michael.henretty@gmail.com>
Desc: Bug 924251 - fixing markup to use building blocks for offline overlay