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

========

https://hg.mozilla.org/integration/gaia-central/rev/75c032886fac
Author: gasolin <gasolin@gmail.com>
Desc: Merge pull request #15135 from gasolin/issue-950766-2

Bug 950766 - [B2G][Window Management] wrapper UI incorrectly shows backg...,r=alive

========

https://hg.mozilla.org/integration/gaia-central/rev/968b19d9ed7c
Author: gasolin <gasolin@gmail.com>
Desc: Bug 950766 - [B2G][Window Management] wrapper UI incorrectly shows background when keyboard is shown