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

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https://hg.mozilla.org/integration/gaia-central/rev/29680b72e627
Author: James Lal <james@lightsofapollo.com>
Desc: Merge pull request #12169 from hotzenklotz/915661_marionette_gallery

Bug 915661 - [Gallery] Add Marionette tests

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https://hg.mozilla.org/integration/gaia-central/rev/eb5797c5fe06
Author: Tom Herold <therold@mozilla.com>
Desc: Bug 915661 - [Gallery] Add Marionette tests