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

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https://hg.mozilla.org/integration/gaia-central/rev/0616cdd6dbb0
Author: Cristian Rodriguez <crdlc@tid.es>
Desc: Merge pull request #13714 from jfkthame/patch-2

Bug 914122: Don't use letter-spacing on grid icons, as it results in po...

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https://hg.mozilla.org/integration/gaia-central/rev/4c8ef1a4de47
Author: jfkthame <jfkthame@gmail.com>
Desc: [bug 914122] Don't use letter-spacing on grid icons, as it results in poor glyph spacing

See bug 914122 comment 40. Using negative letter-spacing gives poor-looking text, because the glyph positions are rounded to whole pixels and therefore fine adjustments to the individual positions cannot be accurately realized; instead, we get a few abrupt, ugly whole-pixel shifts at "random" places within the string.