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

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https://hg.mozilla.org/integration/gaia-central/rev/a43283268da1
Author: Albert <acperez@tid.es>
Desc: Merge pull request #15646 from acperez/bug-951681

Bug 951681 - Single variant build process uses invalid init.json

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https://hg.mozilla.org/integration/gaia-central/rev/7cb7e66e960d
Author: Albert <acperez@tid.es>
Desc: Bug 951681 - Single variant build process uses invalid init.json
Bumping manifests a=b2g-bump