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

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https://hg.mozilla.org/integration/gaia-central/rev/4830c9cbbec3
Author: Julien Wajsberg <felash@gmail.com>
Desc: Bug 988768 - [Sora][Message][MMS] Can't send messages with an attachment between 295 and 299KB r=schung

This changes the default setting in Gaia from 300KB to 295KB. This also
substracts 5KB from the setting taken in Gecko.
Bumping manifests a=b2g-bump