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

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https://hg.mozilla.org/integration/gaia-central/rev/8754e2ededd4
Author: ochameau <poirot.alex@gmail.com>
Desc: Merge pull request #15502 from ochameau/utility-tray

Bug 961514 - Ease dragging the utility tray r=alive

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https://hg.mozilla.org/integration/gaia-central/rev/fc7a524b59a4
Author: Alexandre Poirot <poirot.alex@gmail.com>
Desc: Bug 961514 - Ease dragging the utility tray r=alive
Bumping manifests a=b2g-bump