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

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https://hg.mozilla.org/integration/gaia-central/rev/d2670944b229
Author: Jonathan Griffin <jgriffin@mozilla.com>
Desc: Merge pull request #15849 from jonallengriffin/gaiaunitcrash

Bug 965882 - Add crash handling to gaia_unit runner, r=ahal

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https://hg.mozilla.org/integration/gaia-central/rev/7b251cb18fe1
Author: Jonathan Griffin <jgriffin@mozilla.com>
Desc: Bug 965882 - Add crash handling to gaia_unit runner, r=ahal

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https://hg.mozilla.org/integration/gaia-central/rev/4428b79a734e
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Merge pull request #15611 from KevinGrandon/bug_962655_rocketbar_build_flag

Bug 962655 - Use build-time preference to enable rocketbar

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https://hg.mozilla.org/integration/gaia-central/rev/ba9ab20fccfe
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Bug 962655 - Use build-time preference to enable rocketbar r=vingtetun, yurenju
Bumping manifests a=b2g-bump