Bumping gaia.json for 4 gaia-central revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/adb7896ababd
Author: Cristian Rodriguez <crdlc@tid.es>
Desc: Merge pull request #14209 from crdlc/bug-940290

Bug 940290 - [Download Manager] Implement timestamps for download notifi...

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https://hg.mozilla.org/integration/gaia-central/rev/53ba421475c0
Author: crdlc <crdlc@tid.es>
Desc: Bug 940290 - [Download Manager] Implement timestamps for download notifications

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https://hg.mozilla.org/integration/gaia-central/rev/dae22d6e2cd9
Author: Dominic Kuo <dkuo@mozilla.com>
Desc: Merge pull request #14251 from dominickuo/bug-943797

Bug 943797 - [music] Use hash to decide the default album art, r=jimporter

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https://hg.mozilla.org/integration/gaia-central/rev/d6b776c5cec1
Author: Dominic Kuo <dkuo@mozilla.com>
Desc: Bug 943797 - [music] Use hash to decide the default album art