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

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https://hg.mozilla.org/integration/gaia-central/rev/f7cea851dfa5
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Merge pull request #20764 from KevinGrandon/benfrancis-1026293

Bug 1026293 - Remove e.me as an available search engine and set new default in build

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https://hg.mozilla.org/integration/gaia-central/rev/c6d7ae14921e
Author: Ben Francis <ben@tola.me.uk>
Desc: Bug 1026293 - Remove e.me as an available search engine and set new default in build r=kgrandon

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https://hg.mozilla.org/integration/gaia-central/rev/855aecfb0b4b
Author: Yura Zenevich <yura.zenevich@gmail.com>
Desc: Merge pull request #20502 from yzen/bug-1024720

Bug 1024720 - making accessibility settings' volume and rate sliders vis...

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https://hg.mozilla.org/integration/gaia-central/rev/1043987ad3d8
Author: Yura Zenevich <yzenevich@mozilla.com>
Desc: Bug 1024720 - making accessibility settings' volume and rate sliders visible. Adding gaia-ui tests to avoid future regressions. r=arthurcc, bebe
Bumping manifests a=b2g-bump