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

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https://hg.mozilla.org/integration/gaia-central/rev/74d4979bd169
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Merge pull request #20118 from KevinGrandon/bug_1021510_system_activity_transitions

Bug 1021510 - [System] Activity transitions

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https://hg.mozilla.org/integration/gaia-central/rev/c83aba134f40
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Bug 1021510 - [System] Activity transitions r=alive
Bumping manifests a=b2g-bump