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

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https://hg.mozilla.org/integration/gaia-central/rev/a15feb6893e1
Author: Francisco Jordano <arcturus@ardeenelinfierno.com>
Desc: Merge pull request #21453 from arcturus/bug-1015388

Bug 1015388 - [Contacts] Implement new startup loading events

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https://hg.mozilla.org/integration/gaia-central/rev/8b1bb6e7073b
Author: Francisco Jordano <fjordano@mozilla.com>
Desc: Bug 1015388 - [Contacts] Implement new startup loading events

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https://hg.mozilla.org/integration/gaia-central/rev/3a370d2e385c
Author: Chris Lord <chrislord.net@gmail.com>
Desc: Merge pull request #21704 from Cwiiis/bug1038185-close-utility-tray-transition

Bug 1038185 - Fix transition when closing utility tray. r=etienne

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https://hg.mozilla.org/integration/gaia-central/rev/4de79e0980ef
Author: Chris Lord <chrislord.net@gmail.com>
Desc: Bug 1038185 - Fix transition when closing utility tray. r=etienne
Bumping manifests a=b2g-bump