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

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https://hg.mozilla.org/integration/gaia-central/rev/742a017e771d
Author: Cristian Rodriguez <crdlc@tid.es>
Desc: Merge pull request #12624 from crdlc/bug-912515

Bug 912515 - Update Homescreen and App Grid for Unification and Legibility

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https://hg.mozilla.org/integration/gaia-central/rev/426c794bf0f3
Author: crdlc <crdlc@tid.es>
Desc: Bug 912515 - Update Homescreen and App Grid for Unification and Legibility

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https://hg.mozilla.org/integration/gaia-central/rev/e684dda89893
Author: Evyatar Amitay <evyatron@gmail.com>
Desc: Merge pull request #12638 from EverythingMe/923399-translate-download

[Bug 923399] Translate "Download" label for market apps [r=crdlc]

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https://hg.mozilla.org/integration/gaia-central/rev/5029d347917d
Author: Evyatar Amitay <evyatron@gmail.com>
Desc: [Bug 923399] Translate "Download" label for market apps [r=crdlc]