Bumping gaia.json for 2 gaia-1_2 revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-1_2/rev/af2e781140e3
Author: James Lal <james@lightsofapollo.com>
Desc: Merge pull request #13371 from lightsofapollo/lock-to-v1.2

Bug 929062 - Use b2g-1.2 gecko branch +shepherd

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https://hg.mozilla.org/integration/gaia-1_2/rev/fb9e62b27076
Author: James Lal <james@lightsofapollo.com>
Desc: Bug 929062 - Use b2g-1.2 gecko branch