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

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https://hg.mozilla.org/integration/gaia-central/rev/57b92592d977
Author: Anthony Ricaud <anthony@ricaud.me>
Desc: Merge pull request #15109 from Rik/conference-error-938099

Bug 938099 - display conference error when merging or removing r=etienne

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https://hg.mozilla.org/integration/gaia-central/rev/c1342803ec71
Author: Anthony Ricaud <anthony@ricaud.me>
Desc: Bug 938099 - display conference error when merging or removing