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

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https://hg.mozilla.org/integration/gaia-central/rev/89c57a31cebe
Author: Casey Yee <cyee@mozilla.com>
Desc: Merge pull request #13298 from caseyyee/bug-925929

Updated headers building blocks to 1.3 visual design styling

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https://hg.mozilla.org/integration/gaia-central/rev/0d02e40bf07b
Author: Casey Yee <casey@caseyyee.com>
Desc: Bug 925929: [Building Blocks] Update headers BB visual design

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https://hg.mozilla.org/integration/gaia-central/rev/343a257bbba7
Author: Yuren Ju <yurenju@gmail.com>
Desc: Merge pull request #13792 from yurenju/minilla-homescreen

Bug 897325 - Part 1: Create web UI for homescreen customization r=ochameau

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https://hg.mozilla.org/integration/gaia-central/rev/e6a2a27ce950
Author: Yuren Ju <yurenju@gmail.com>
Desc: Bug 897325 - Part 1: Create web UI for homescreen customization