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

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https://hg.mozilla.org/integration/gaia-central/rev/5874a36abde8
Author: Alive.Kuo <alegnadise@gmail.com>
Desc: Merge pull request #16255 from desaintmartin/972475

Bug 972475 - [System] Fix removal of event listener.r=alive

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https://hg.mozilla.org/integration/gaia-central/rev/fafe7adf2428
Author: Cédric de Saint Martin <cedric.dsm@tiolive.com>
Desc: Bug 972475 - [System] Fix removal of event listener.
Bumping manifests a=b2g-bump