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

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https://hg.mozilla.org/integration/gaia-central/rev/dc6ffbf77d6c
Author: Ryan VanderMeulen <rvandermeulen@mozilla.com>
Desc: Merge pull request #16385 from flodolo/bug973787

Bug 973787 - [Gaia] SD card formatting: reword message and buttons

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https://hg.mozilla.org/integration/gaia-central/rev/d12ac838cc74
Author: Francesco Lodolo (:flod) <flod@lodolo.net>
Desc: Bug 973787 - [Gaia] SD card formatting: reword message and appearance of the format button

New main string suggested by :matej

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https://hg.mozilla.org/integration/gaia-central/rev/462fe11800bf
Author: Ryan VanderMeulen <rvandermeulen@mozilla.com>
Desc: Merge pull request #16430 from theres-waldo/master

Bug 964517 - Add solid background to apps where appropriate. r=alive

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https://hg.mozilla.org/integration/gaia-central/rev/5df30f69d655
Author: Botond Ballo <bballo@mozilla.com>
Desc: Bug 964517 - Add solid background to apps where appropriate. r=alive
Bumping manifests a=b2g-bump