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

========

https://hg.mozilla.org/integration/gaia-central/rev/6bc990a70f2f
Author: gasolin <gasolin@gmail.com>
Desc: Merge pull request #16192 from gasolin/issue-971397

Bug 971397 - [customization] usage guide referenced in README is moved t..., r=yurenju

========

https://hg.mozilla.org/integration/gaia-central/rev/b237fdefb2d2
Author: gasolin <gasolin@gmail.com>
Desc: Bug 971397 - [customization] usage guide referenced in README is moved to MDN
Bumping manifests a=b2g-bump