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

========

https://hg.mozilla.org/integration/gaia-central/rev/900a68659c7c
Author: Eitan Isaacson <eitan@monotonous.org>
Desc: Merge pull request #12783 from yzen/916282

Bug 916282 -  Making lock and camera buttons available to explore by tou...

========

https://hg.mozilla.org/integration/gaia-central/rev/f9b2033bc6d8
Author: Yura Zenevich <yura.zenevich@gmail.com>
Desc: Bug 916282 - Making lock and camera buttons available to explore by touch in screen reader.