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

========

https://hg.mozilla.org/integration/gaia-central/rev/6215881c830f
Author: Guillaume C. Marty <gmarty@users.noreply.github.com>
Desc: Merge pull request #26608 from gmarty/Bug-998328-update-gaia-icons

Bug 998328 - Update gaia-icons to the latest version

========

https://hg.mozilla.org/integration/gaia-central/rev/da4f3e00edd3
Author: Guillaume Marty <edo999@gmail.com>
Desc: Bug 998328 - Update gaia-icons to the latest version
Bumping manifests a=b2g-bump