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

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https://hg.mozilla.org/integration/gaia-central/rev/c60915db4edd
Author: Marcus Cavanaugh <m@mcav.com>
Desc: Merge pull request #18426 from mcav/attachments-suck

Bug 996232 - [B2G][Email] Downloading large attachments will result... r=asuth

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https://hg.mozilla.org/integration/gaia-central/rev/9c0f7421179a
Author: Marcus Cavanaugh <m@mcav.com>
Desc: Bug 996232 - [B2G][Email] Downloading large attachments will result in the e-mail app being killed. r=asuth
Bumping manifests a=b2g-bump