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

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https://hg.mozilla.org/integration/gaia-central/rev/e551202b0e63
Author: Arthur Chen <crh0716@gmail.com>
Desc: Merge pull request #15152 from crh0716/943210

Bug 943210 - Use SIMSlotManager r=alive

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https://hg.mozilla.org/integration/gaia-central/rev/4eb4b229007b
Author: Arthur Chen <crh0716@gmail.com>
Desc: Bug 943210 - Use SIMSlotManager

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https://hg.mozilla.org/integration/gaia-central/rev/506efcf2c188
Author: Zac <zcampbell@mozilla.com>
Desc: Merge pull request #15159 from viorelaioia/bug_958074

Bug 958074 - Remove unused import in test_sms_notification

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https://hg.mozilla.org/integration/gaia-central/rev/7eaa6f93f941
Author: Viorela Ioia <viorelaioia@gmail.com>
Desc: Bug 958074 - Remove unused import in test_sms_notification