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

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https://hg.mozilla.org/integration/gaia-central/rev/f6f928dc9106
Author: Ben Kelly <bkelly@mozilla.com>
Desc: Merge pull request #13593 from wanderview/contacts-group-overflow

Bug 937291:  Limit overflow of each group list.  r=jmcf

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https://hg.mozilla.org/integration/gaia-central/rev/f5b424f68347
Author: Ben Kelly <ben@wanderview.com>
Desc: Bug 937291:  Limit overflow of each group list.