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

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https://hg.mozilla.org/integration/gaia-central/rev/55d5c08c9665
Author: Trif Andrei-Alin <trifandreialin@gmail.com>
Desc: Merge pull request #14028 from bobsilverberg/bug932753

Bug 932753 - Update test_settings_do_not_track to use new Gecko pref helper methods

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https://hg.mozilla.org/integration/gaia-central/rev/1af23c081d0b
Author: Bob Silverberg <bob.silverberg@gmail.com>
Desc: Bug 932753 - Update test_settings_do_not_track to use new Gecko pref helper methods

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https://hg.mozilla.org/integration/gaia-central/rev/6b43014b5131
Author: Pavel Ivanov <pivanov@mozilla.com>
Desc: Merge pull request #13979 from pivanov/bug-939144

Bug 939144 - [Action Icons - Communications] Remove shadows from action icons

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https://hg.mozilla.org/integration/gaia-central/rev/e1a6ecdcb11b
Author: Pavel Ivanov <pivanov@mozilla.com>
Desc: Bug 939144 - [Action Icons - Communications] Remove shadows from action icons