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

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https://hg.mozilla.org/integration/gaia-central/rev/50e6f76436eb
Author: Timothy Guan-tin Chien <timdream@gmail.com>
Desc: Merge pull request #21654 from timdream/keyboard-alt-sublayout

Bug 1037942 - Normalize alt in overwrite alternative layouts, r=rudyl

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https://hg.mozilla.org/integration/gaia-central/rev/f54018b582b3
Author: Timothy Guan-tin Chien <timdream@gmail.com>
Desc: Bug 1037942 - Normalize alt in overwrite alternative layouts
Bumping manifests a=b2g-bump