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

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https://hg.mozilla.org/integration/gaia-central/rev/d15adbd5c5b1
Author: gasolin <gasolin@gmail.com>
Desc: Merge pull request #14526 from gasolin/issue-948283

Bug 948283 - [BB][refactor] host in single style folder, r=arnau

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https://hg.mozilla.org/integration/gaia-central/rev/59be55b90b5a
Author: gasolin <gasolin@gmail.com>
Desc: Bug 948283 - [BB][refactor] host in single style folder
Bumping manifests a=b2g-bump