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

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https://hg.mozilla.org/integration/gaia-central/rev/ef4d3e1c1bde
Author: Sam Joch <samuel@guiora.com>
Desc: Merge pull request #13359 from pivanov/bug-930546

Bug 930546 - [Media] Replace Gradient Background with Solid Colour r=samjoch

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https://hg.mozilla.org/integration/gaia-central/rev/3fd0de1b7204
Author: Pavel Ivanov <pivanov@mozilla.com>
Desc: Bug 930546 - [Media] Replace Gradient Background with Solid Colour