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

========

https://hg.mozilla.org/integration/gaia-central/rev/9b4d3f14208d
Author: gasolin <gasolin@gmail.com>
Desc: Merge pull request #17495 from gasolin/sheriff976986

Merge pull request #16947 from dwi2/bug976986, r=timdream

========

https://hg.mozilla.org/integration/gaia-central/rev/61d7ab0cfc79
Author: dwi2 <dwi2@dwi2.com>
Desc: Merge pull request #16947 from dwi2/bug976986

Bug 976986 - SettingsListener should have unobserve() method
Bumping manifests a=b2g-bump