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

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https://hg.mozilla.org/integration/gaia-central/rev/5a1b6d990526
Author: George <georgeiscoming@gmail.com>
Desc: Merge pull request #17179 from cctuan/979827

Bug 979827 - [Build] check node version of environment

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https://hg.mozilla.org/integration/gaia-central/rev/6d4ae7fdf48a
Author: cctuan <georgeiscoming@gmail.com>
Desc: Bug 979827 - [Build] check node version of environment
Bumping manifests a=b2g-bump