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

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https://hg.mozilla.org/integration/gaia-central/rev/3942b0cac7bc
Author: Timothy Guan-tin Chien <timdream@gmail.com>
Desc: Merge pull request #13795 from timdream/lockscreen-unlock-transition2

Bug 937630 - All you need is ... smooth unlock transition, r=vingtetun

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https://hg.mozilla.org/integration/gaia-central/rev/88a74c4fa8db
Author: Timothy Guan-tin Chien <timdream@gmail.com>
Desc: Bug 937630 - All you need is ... smooth unlock transition

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https://hg.mozilla.org/integration/gaia-central/rev/b151bb1920b0
Author: Pavel Ivanov <pivanov@mozilla.com>
Desc: Merge pull request #13781 from pivanov/bug-939149

Bug 939149 - [Action Icons - FM Radio] Remove shadows from action icons

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https://hg.mozilla.org/integration/gaia-central/rev/c0a188ab39f9
Author: Pavel Ivanov <pivanov@mozilla.com>
Desc: Bug 939149 - [Action Icons - FM Radio] Remove shadows from action icons