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

========

https://hg.mozilla.org/integration/gaia-central/rev/503fd66f99c2
Author: Alive.Kuo <alegnadise@gmail.com>
Desc: Merge pull request #17270 from alivedise/bugzilla/984299_master/keyboard-close-itself-on-hidden

Bug 984299 - Use page visibility to close keyboard, r=rudy

========

https://hg.mozilla.org/integration/gaia-central/rev/557f5cd602d3
Author: Alive Kuo <alegnadise@gmail.com>
Desc: Bug 984299 - Use page visibility to close keyboard
Bumping manifests a=b2g-bump