Bumping gaia.json for 1 gaia-1_2 revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-1_2/rev/3a7f5bc63306
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
(cherry picked from commit bdfb965ead3c0b0aa63c9b0394632fcadd73e6ba)