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

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https://hg.mozilla.org/integration/gaia-central/rev/51ea613dbd11
Author: Zac <zcampbell@mozilla.com>
Desc: Merge pull request #12861 from bobsilverberg/e.me-wait

Bug 926971 - Use wait_for_everything_me_loaded in e.me tests, r=zac

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https://hg.mozilla.org/integration/gaia-central/rev/4dcf36a46f46
Author: Bob Silverberg <bob.silverberg@gmail.com>
Desc: Bug 926971 - Use wait_for_everything_me_loaded in e.me tests, r=zac