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

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https://hg.mozilla.org/integration/gaia-central/rev/e215aeda607a
Author: Anthony Ricaud <anthony@ricaud.me>
Desc: Bug 935904 - Mark the test as intermittent r=me

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https://hg.mozilla.org/integration/gaia-central/rev/f21c89055703
Author: Anthony Ricaud <anthony@ricaud.me>
Desc: Bug 932331 - Mark the test as intermittent r=me

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https://hg.mozilla.org/integration/gaia-central/rev/6aa87e4cddc0
Author: Pavel Ivanov <pivanov@mozilla.com>
Desc: Merge pull request #13982 from pivanov/bug-939219

Bug 939219 - [Action Icons - Music] Remove shadows from action icons

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https://hg.mozilla.org/integration/gaia-central/rev/449f93c9a872
Author: Pavel Ivanov <pivanov@mozilla.com>
Desc: Bug 939219 - [Action Icons - Music] Remove shadows from action icons