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

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https://hg.mozilla.org/integration/gaia-central/rev/4523c8a627a4
Author: gasolin <gasolin@gmail.com>
Desc: Merge pull request #17378 from desaintmartin/971556_system2_sourceview_master

Bug 971556 - [System2] Instantiable SourceView, r=alive

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https://hg.mozilla.org/integration/gaia-central/rev/b9f7db953eb6
Author: Cédric de Saint Martin <cedric.dsm@tiolive.com>
Desc: Bug 971556 - [System2] Instantiable SourceView.
Bumping manifests a=b2g-bump