Bumping gaia.json for 3 gaia-central revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/2b4977c69cb4
Author: Rex KM Lee <rexboy@mozilla.com>
Desc: Merge pull request #15032 from rexboy7/fix-bug900801

bug 900801 - [Dialer] Allow switching audio source between BT / receiver / speaker, r=etienne

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https://hg.mozilla.org/integration/gaia-central/rev/113252d7a49c
Author: Rex Lee <rexboy@mozilla.com>
Desc: Bug 900801 - (Revise 1 for reviewing)

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https://hg.mozilla.org/integration/gaia-central/rev/584a8d8ae93a
Author: Rex Lee <rexboy@mozilla.com>
Desc: bug 900801 - [Dialer] Allow switching audio source between BT / receiver / speaker