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

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https://hg.mozilla.org/integration/gaia-central/rev/aca46d5318c9
Author: James Burke <jrburke@gmail.com>
Desc: Merge pull request #15829 from jrburke/bug882000-email-download-button

Bug 882000 - [Email] Attachment icons are not vertically center aligned r=mcav

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https://hg.mozilla.org/integration/gaia-central/rev/b3e70c0527c2
Author: jrburke <jrburke@gmail.com>
Desc: Bug 882000 - [Email] Attachment icons are not vertically center aligned
Bumping manifests a=b2g-bump