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

========

https://hg.mozilla.org/integration/gaia-central/rev/56d9b9b0a30a
Author: Yoshi Huang <allstars.chh@mozilla.com>
Desc: Merge pull request #18740 from allstarschh/Bug996522

Bug 996522 - [NFC] reset currentURL to null in Homescreen. r=alive

========

https://hg.mozilla.org/integration/gaia-central/rev/74fa39dfcc21
Author: Yoshi Huang <allstars.chh@mozilla.com>
Desc: Bug 996522 - [NFC] reset currentURL to null in Homescreen. r=alive
Bumping manifests a=b2g-bump