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

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https://hg.mozilla.org/integration/gaia-central/rev/b8903ca53b5b
Author: Carsten Book <tomcat@mozilla.com>
Desc: Merge pull request #18895 from azasypkin/bug-996900-keyboard-on-click

Bug 996900 - [Messaging] Bring up keyboard when user taps in the middle of the Messaging screen. r=julien

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https://hg.mozilla.org/integration/gaia-central/rev/9746f3d86701
Author: Aleh Zasypkin <azasypkin@mozilla.com>
Desc: Bug 996900 - [Messaging] Bring up keyboard when user taps in the middle of the Messaging screen. r=julien
Bumping manifests a=b2g-bump