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

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https://hg.mozilla.org/integration/gaia-central/rev/607477b163d7
Author: Arthur Chen <crh0716@gmail.com>
Desc: Merge pull request #15351 from crh0716/956428

Bug 956428 - Add checks on set/getPreferredNetworkType r=jaoo

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https://hg.mozilla.org/integration/gaia-central/rev/adae252a0976
Author: Arthur Chen <crh0716@gmail.com>
Desc: Bug 956428 - Add checks on set/getPreferredNetworkType

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https://hg.mozilla.org/integration/gaia-central/rev/b10d268fa47c
Author: lissyx <lissyx+github@lissyx.dyndns.org>
Desc: Merge pull request #15307 from lissyx/bug855165

Bug 855165 - Migrate to new Notification API and close notifications r=rwaldron

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https://hg.mozilla.org/integration/gaia-central/rev/87f577156f3d
Author: Alexandre Lissy <lissyx+github@lissyx.dyndns.org>
Desc: Bug 855165 - Migrate Messages to new Notification API and close notifications upon reading of threads