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

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https://hg.mozilla.org/integration/gaia-central/rev/18117c1ee38f
Author: ochameau <poirot.alex@gmail.com>
Desc: Merge pull request #21281 from ochameau/rm

Bug 1033384 - Fix gaia buttons in responsive mode. r=21

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https://hg.mozilla.org/integration/gaia-central/rev/9f72065d69e4
Author: Alexandre Poirot <poirot.alex@gmail.com>
Desc: Bug 1033384 - Fix gaia button in responsive mode. r=21
Bumping manifests a=b2g-bump