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

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https://hg.mozilla.org/integration/gaia-central/rev/6497ac39da4b
Author: Julien Wajsberg <felash@gmail.com>
Desc: Merge pull request #14673 from julienw/950083-css-changes

Bug 950083 - css changes

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https://hg.mozilla.org/integration/gaia-central/rev/bc9b96cfe376
Author: Julien Wajsberg <felash@gmail.com>
Desc: Bug 950083 - CSS fixes after bug 943807 landed r=yurenju

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https://hg.mozilla.org/integration/gaia-central/rev/389cc8c11491
Author: Cristian Rodriguez <crdlc@tid.es>
Desc: Merge pull request #14634 from crdlc/bug-949948

Bug 949948 - [Homescreen] Dock tests fail when it is launched isolated

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https://hg.mozilla.org/integration/gaia-central/rev/9d19d6e17075
Author: crdlc <crdlc@tid.es>
Desc: Bug 949948 - [Homescreen] Dock tests fail when it is launched isolated