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

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https://hg.mozilla.org/integration/gaia-central/rev/b6699ab6214f
Author: Ian Liu <iliu@mozilla.com>
Desc: Merge pull request #13687 from ian-liu/l10n/Bug936812_Missing_strings_in_en-US_properties_bluetooth_dialer_system

Bug 936812 - Missing strings in en-US properties (bluetooth, dialer, system), r=@etiennesegonzac

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https://hg.mozilla.org/integration/gaia-central/rev/7a052b4eb55a
Author: ian-liu <iliu@mozilla.com>
Desc: Bug 936812 - Missing strings in en-US properties (bluetooth, dialer, system)

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https://hg.mozilla.org/integration/gaia-central/rev/6987a03e5632
Author: Germán Toro del Valle <gtorodelvalle@gmail.com>
Desc: Merge pull request #13712 from gtorodelvalle/contacts-bug-937040-removing-and-saving-mozContacts

Bug 937040 - Revise contacts app to adapt to WebIDL changes in a uniform way

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https://hg.mozilla.org/integration/gaia-central/rev/8354c35440be
Author: Germán Toro del Valle <gtorodelvalle@gmail.com>
Desc: Bug 937040 - Revise contacts app to adapt to WebIDL changes in a uniform way