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

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https://hg.mozilla.org/integration/gaia-central/rev/178492048ae0
Author: Rudy Lu <me@rudy.lu>
Desc: Merge pull request #13977 from alefteris/bug-942580-kb-final-sigma

Bug 942580 - Remove duplicate "final sigma" from Greek keyboard.
r=rudylu

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https://hg.mozilla.org/integration/gaia-central/rev/a184e2e69804
Author: Thanos Lefteris <alefteris@gmail.com>
Desc: Bug 942580 - Remove dublicate "final sigma" from Greek keyboard