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

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https://hg.mozilla.org/integration/gaia-central/rev/eb577f805b15
Author: Ryan VanderMeulen <rvandermeulen@mozilla.com>
Desc: Merge pull request #12888 from yxl/keyboard-switch

Bug 927302 - [InputMethod] Cannot switch keyboard from FF Nightly.

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https://hg.mozilla.org/integration/gaia-central/rev/73b462347a7a
Author: Yuan Xulei(袁徐磊) <xyuan@mozilla.com>
Desc: Bug 927302 - [InputMethod] Cannot switch keyboard from FF Nightly. r=janjongboom

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https://hg.mozilla.org/integration/gaia-central/rev/0767b22c5c37
Author: Sam Joch <samuel@guiora.com>
Desc: Merge pull request #12350 from pivanov/bug-918872

Bug 918872 - System screen should use full highlights r=samjoch

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https://hg.mozilla.org/integration/gaia-central/rev/4230f9acb410
Author: Pavel Ivanov <pivanov@mozilla.com>
Desc: Bug 918872 - System screen should use full highlights