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

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https://hg.mozilla.org/integration/gaia-central/rev/57cb6d635bad
Author: Borja Salguero <borjasalguero@users.noreply.github.com>
Desc: Merge pull request #16572 from borjasalguero/composer_flex

Bug 949457 - Make Compose into a flex layout r=julienw

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https://hg.mozilla.org/integration/gaia-central/rev/d5824ae76962
Author: borjasalguero <fbsc@tid.es>
Desc: Bug 949457 - Make Compose into a flex layout r=julienw
Bumping manifests a=b2g-bump