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

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https://hg.mozilla.org/integration/gaia-central/rev/c5a892f84686
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Merge pull request #15415 from KevinGrandon/revert_pr_15065

Revert "Bug 957100 - [Settings] add marionette for b2g-improve and feedback page"

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https://hg.mozilla.org/integration/gaia-central/rev/f67de4427dd1
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Revert "Bug 957100 - [Settings] add marionette for b2g-improve and feedback page"

This reverts commit 0b49a29ae694f55333ab2a0054d85597a3e5582d.

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https://hg.mozilla.org/integration/gaia-central/rev/4a33e55f12f2
Author: Etienne Segonzac <etienne@segonzac.info>
Desc: Merge pull request #15403 from etiennesegonzac/bug-849223-css-system-settings

Bug 849223 - Adding default values for the gaia.ui.* theming settings

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https://hg.mozilla.org/integration/gaia-central/rev/e3a9e471ab5a
Author: Etienne Segonzac <etienne@segonzac.info>
Desc: Bug 849223 - Adding default values for the gaia.ui.* theming settings
r=vingtetun