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

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https://hg.mozilla.org/integration/gaia-central/rev/3981aa44b3b0
Author: Gareth Aye <gaye@mozilla.com>
Desc: Merge pull request #17088 from gaye/bug-974731

Bug 974731 - Failing calendar marionette test create_event_test.js

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https://hg.mozilla.org/integration/gaia-central/rev/75405ef01833
Author: Gareth Aye <gaye@mozilla.com>
Desc: Bug 974731 - Failing calendar marionette test create_event_test.js
Bumping manifests a=b2g-bump