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

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https://hg.mozilla.org/integration/gaia-central/rev/d838e2c87b76
Author: Etienne Segonzac <etienne@segonzac.info>
Desc: Merge pull request #14892 from etiennesegonzac/bug-948975-resume-single-conference-group

Bug 948975 - Safeguard to make sure we can always resume a single held conference call. r=rik

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https://hg.mozilla.org/integration/gaia-central/rev/884f77e61fa4
Author: Etienne Segonzac <etienne@segonzac.info>
Desc: Bug 948975 - Safeguard to make sure we can always resume a single held
conference call.