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

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https://hg.mozilla.org/integration/gaia-central/rev/2b7f5c48bcc5
Author: Tzu-Lin Huang <dwi019@gmail.com>
Desc: Merge pull request #25863 from dwi2/bug1014418_v2

Bug 1014418 - Dispatching hardware button event in Gaia, r=timdream

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https://hg.mozilla.org/integration/gaia-central/rev/7bab6e845e1a
Author: Tzu-Lin Huang <tzhuang@mozilla.com>
Desc: Bug 1014418 - Dispatching hardware button event in Gaia
Bumping manifests a=b2g-bump