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

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https://hg.mozilla.org/integration/gaia-central/rev/73de68772ff4
Author: Zac <zcampbell@mozilla.com>
Desc: Merge pull request #16952 from viorelaioia/bug_979789

Bug 979789 - Check and enable some more tests on TBPL

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https://hg.mozilla.org/integration/gaia-central/rev/76c25f11d328
Author: Viorela Ioia <viorelaioia@gmail.com>
Desc: Bug 979789 - enable test_sms_contact_input_validation, test_power_button_long_press and test_window_open_inside_iframe
Bumping manifests a=b2g-bump