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

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https://hg.mozilla.org/integration/gaia-central/rev/57f2289efede
Author: Ryan VanderMeulen <rvandermeulen@mozilla.com>
Desc: Merge pull request #18433 from yzen/bug-997900

Bug 997900 - fixed screen reader visibility of the status bar icons when...

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https://hg.mozilla.org/integration/gaia-central/rev/127ed19ee987
Author: Yura Zenevich <yzenevich@mozilla.com>
Desc: Bug 997900 - fixed screen reader visibility of the status bar icons when the rocket bar is active. r=benfrancis

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https://hg.mozilla.org/integration/gaia-central/rev/17390a4a348c
Author: Douglas Sherk <github@sherk.me>
Desc: Merge pull request #18756 from DouglasSherk/998972-voicemail-sim-indicator-string

Bug 998972 - [System] Use a new string for voicemail SIM notifications. r=etienne

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https://hg.mozilla.org/integration/gaia-central/rev/2f1ef5e1b0ed
Author: DouglasSherk <github@sherk.me>
Desc: Bug 998972 - [System] Use a new string for voicemail SIM notifications. r=etienne
Bumping manifests a=b2g-bump