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

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https://hg.mozilla.org/integration/gaia-central/rev/a2618be50b16
Author: Julien Wajsberg <felash@gmail.com>
Desc: Merge pull request #24416 from julienw/1062323-fix-promise-chain-voicemail

Bug 1062323 - Promise sequencing incorrect - fix voicemail test r=etienn...

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https://hg.mozilla.org/integration/gaia-central/rev/76b7ea0d9703
Author: Julien Wajsberg <felash@gmail.com>
Desc: Bug 1062323 - Promise sequencing incorrect - fix voicemail test r=etienne
Bumping manifests a=b2g-bump