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

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https://hg.mozilla.org/integration/gaia-2_0/rev/8ac857502439
Author: Rudy Lu <me@rudy.lu>
Desc:  Bug 998106 - [B2G][Gaia][FTE][Keyboard] Selecting languages other than
 English during the FTE causes the associated keyboard not to load.
  - Filter out "language to layouts mappings" if the layout is not
    preloaded.
Bumping manifests a=b2g-bump