Bumping gaia.json for 2 gaia-central revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/4a20fc3b7752
Author: Zac <zcampbell@mozilla.com>
Desc: Merge pull request #15098 from bebef1987/bug-957545

Bug 957545 - Disable test_dialer_add_contact because it's erroring on every run

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https://hg.mozilla.org/integration/gaia-central/rev/76a28e56bfb3
Author: Bebe <bebe@mozilla.ro>
Desc: Bug 957545 - Disable test_dialer_add_contact because it's erroring on every run