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

========

https://hg.mozilla.org/integration/gaia-central/rev/a15a2479ef80
Author: Joan Leon <joan.leon@gmail.com>
Desc: Merge pull request #17386 from nucliweb/973827-Power-button-modal

Power-button-modal

========

https://hg.mozilla.org/integration/gaia-central/rev/37cfe1536eaf
Author: Joan Leon <joan.leon@gmail.com>
Desc: Power-button-modal
Bumping manifests a=b2g-bump