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

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https://hg.mozilla.org/integration/gaia-central/rev/a19581a56f75
Author: Pavel Ivanov <pivanov@mozilla.com>
Desc: Merge pull request #16918 from pivanov/bug-979696

Folowup patch for Bug 979696

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https://hg.mozilla.org/integration/gaia-central/rev/87eec8bd392a
Author: Pavel Ivanov <pivanov@mozilla.com>
Desc: Folowup patch for Bug 979696

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https://hg.mozilla.org/integration/gaia-central/rev/73bd440eef89
Author: Pavel Ivanov <pivanov@mozilla.com>
Desc: Merge pull request #16908 from pivanov/bug-979696

Bug 979696 - Update Photographic Wallpapers Sizes for 1.4

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https://hg.mozilla.org/integration/gaia-central/rev/cca85e9f8e17
Author: Pavel Ivanov <pivanov@mozilla.com>
Desc: Bug 979696 - Update Photographic Wallpapers Sizes for 1.4
Bumping manifests a=b2g-bump