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

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https://hg.mozilla.org/integration/gaia-central/rev/28e5e24c95d9
Author: Etienne Segonzac <etienne@segonzac.info>
Desc: Merge pull request #12749 from etiennesegonzac/bug-910584-dialer-headset-and-speaker

Bug 910584 - Plugging headphones in during a call should turn off the speaker if it's on. r=Rik

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https://hg.mozilla.org/integration/gaia-central/rev/fbad3d76b9e8
Author: Etienne Segonzac <etienne@segonzac.info>
Desc: Bug 910584 - Plugging headphones in during a call should turn off the
speaker if it's on.
r=Rik