Bumping gaia.json for 2 gaia-1_2 revision(s) a=gaia-bump

========

https://hg.mozilla.org/integration/gaia-1_2/rev/ef38fd18fdfb
Author: Timothy Guan-tin Chien <timdream@gmail.com>
Desc: Bug 906549 - Set a background-size to handle images

========

https://hg.mozilla.org/integration/gaia-1_2/rev/6768238cb886
Author: Greg Weng <snowmantw@gmail.com>
Desc: Merge pull request #13063 from snowmantw/issue906549

Bug 906549 - [Flatfish][LockScreen] add support for large device for portrait