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

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https://hg.mozilla.org/integration/gaia-central/rev/6d9dda570404
Author: Zac <zcampbell@mozilla.com>
Desc: Merge pull request #13121 from anah/bug921424

Bug 921424 - Replace wait for landing page after unlocking with wait for displayed app to be homescreen

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https://hg.mozilla.org/integration/gaia-central/rev/ba50c07ee49a
Author: Ana Hristova <ana.nikolaeva@gmail.com>
Desc: Bug 921424 - Replace wait for landing page after unlocking with wait for displayed app to be homescreen