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

========

https://hg.mozilla.org/integration/gaia-central/rev/c58a78c93c48
Author: gasolin <gasolin@gmail.com>
Desc: Merge pull request #12914 from gasolin/issue-927749-2

Bug 927749 - [Flatfish][Homescreen] hide add collection menu option when..., r=crdlc

========

https://hg.mozilla.org/integration/gaia-central/rev/a54120656789
Author: gasolin <gasolin@gmail.com>
Desc: Bug 927749 - [Flatfish][Homescreen] hide add collection menu option when searchbar is hidden