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

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https://hg.mozilla.org/integration/gaia-central/rev/c9b5d8915393
Author: Ben Francis <ben@tola.me.uk>
Desc: Merge pull request #18204 from daleharvey/992884

Bug 992884 - Implement browser start page. r=benfrancis

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https://hg.mozilla.org/integration/gaia-central/rev/8036790fe016
Author: Dale Harvey <dale@arandomurl.com>
Desc: Bug 992884 - Implement browser start page. r=benfrancis
Bumping manifests a=b2g-bump