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

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https://hg.mozilla.org/integration/gaia-central/rev/4e8959a60a4e
Author: Anthony Ricaud <anthony@ricaud.me>
Desc: Merge pull request #13794 from Rik/conference-call-hold-925359

Bug 925359 - Tapping on the Group call layer holds the call r=etienne

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https://hg.mozilla.org/integration/gaia-central/rev/beee198737cc
Author: Anthony Ricaud <anthony@ricaud.me>
Desc: Bug 925359 - Tapping on the Group call layer holds the call