Bumping gaia.json for 3 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-2_1/rev/276496adab76
Author: albertopq <apastor@mozilla.com>
Desc: Bug 1061854 - Dismiss notifications

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https://hg.mozilla.org/integration/gaia-2_1/rev/00085798a7f2
Author: Dale Harvey <dale@arandomurl.com>
Desc: Bug 1053261 - Add customisable preloaded top sites. r=kgrandon

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https://hg.mozilla.org/integration/gaia-2_1/rev/9de608f191ff
Author: Alive Kuo <alegnadise@gmail.com>
Desc: Bug 1063161 - Double alert in activity chain
Bumping manifests a=b2g-bump