Bumping gaia.json for 1 gaia revision(s) a=gaia-bump

========

https://hg.mozilla.org/integration/gaia-central/rev/f6900c83dada
Author: Ryan VanderMeulen <rvandermeulen@mozilla.com>
Desc: Revert "Merge pull request #21423 from TheoChevalier/bug-1028808-inconsistency-homescreen"

This reverts commit b41c4a8f90e80501a4095c232e79016b2bc196ea, reversing
changes made to 6e651273309d69c0f222cfbceee53b741b91b9bd.
Bumping manifests a=b2g-bump