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

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https://hg.mozilla.org/integration/gaia-central/rev/d6287476a7c3
Author: gasolin <gasolin@gmail.com>
Desc: Merge pull request #21785 from gasolin/issue-1033921

Bug 1033921 - [NFC] Shrinking UI incorrectly when using swipe gesture to..., r=alive

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https://hg.mozilla.org/integration/gaia-central/rev/53de4ac7a977
Author: gasolin <gasolin@gmail.com>
Desc: Bug 1033921 - [NFC] Shrinking UI incorrectly when using swipe gesture to change apps
- listen to appwill-become-active
Bumping manifests a=b2g-bump