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

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https://hg.mozilla.org/integration/gaia-central/rev/d57b5177ef36
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Merge pull request #15226 from KevinGrandon/bug_952629_reenable_rocketbar_tests

Bug 952629 - [Rocketbar] Re-enable marionette tests r=me

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https://hg.mozilla.org/integration/gaia-central/rev/d50fbe1b5048
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Bug 952629 - [Rocketbar] Re-enable marionette tests