Bumping gaia.json for 4 gaia-central revision(s) a=gaia-bump

========

https://hg.mozilla.org/integration/gaia-central/rev/af2c025bbb50
Author: Jose M. Cantera <jmcanterafonseca@gmail.com>
Desc: Merge pull request #12431 from jmcanterafonseca/bug-918674

Bug 918674 - In Contacts header the separator between '+' and 'settings ...

========

https://hg.mozilla.org/integration/gaia-central/rev/3238bfb629bf
Author: Jose M. Cantera <jmcanterafonseca@gmail.com>
Desc: Bug 918674 - In Contacts header the separator between '+' and 'settings icon' has disappeared

========

https://hg.mozilla.org/integration/gaia-central/rev/9fdcb2d625f1
Author: gasolin <gasolin@gmail.com>
Desc: Merge pull request #12359 from gasolin/issue-919455

Bug 919455 - tablet wallpaper are stretched, r=yurenju

========

https://hg.mozilla.org/integration/gaia-central/rev/05ecb8099fd7
Author: gasolin <gasolin@gmail.com>
Desc: Bug 919455 - tablet wallpaper are stretched