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

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https://hg.mozilla.org/integration/gaia-central/rev/756725dbbb50
Author: Gareth Aye <gaye@mozilla.com>
Desc: Merge pull request #15587 from gaye/email-mass-reenable

Bug 962959 - Fix and reenable broken or disabled marionette tests in email

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https://hg.mozilla.org/integration/gaia-central/rev/37581d4ad8b4
Author: Gareth Aye <gaye@mozilla.com>
Desc: Bug 962959 - Fix and reenable broken or disabled marionette tests in email
Bumping manifests a=b2g-bump