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

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https://hg.mozilla.org/integration/gaia-central/rev/3bf11691dd7c
Author: Arnau <arnau@arnaumarch.com>
Desc: Merge pull request #15862 from rnowm/966217

Bug 966217 - [VDR] Change bottom background where buttons sit

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https://hg.mozilla.org/integration/gaia-central/rev/758c821a561e
Author: rnowm <arnau@arnaumarch.com>
Desc: Bug 966217 - [VDR] Change bottom background where buttons sit

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https://hg.mozilla.org/integration/gaia-central/rev/1d3ec6e9f932
Author: Casey Yee <cyee@mozilla.com>
Desc: Merge pull request #15830 from caseyyee/bug-962366

Bug 962366 - Replace software home button with something that is not the...

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https://hg.mozilla.org/integration/gaia-central/rev/b5962b7c7de2
Author: Casey Yee <casey@caseyyee.com>
Desc: Bug 962366 - Replace software home button with something that is not the browser icon
Bumping manifests a=b2g-bump