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

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https://hg.mozilla.org/integration/gaia-central/rev/b31738827cc8
Author: Ryan VanderMeulen <rvandermeulen@mozilla.com>
Desc: Merge pull request #17963 from svic/Bug_969468

Bug 969468: Enable NFC preference by default.

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https://hg.mozilla.org/integration/gaia-central/rev/df0fc3edd6ed
Author: Garner Lee <garner.lee@telekom.com>
Desc: Bug 969468: Enable NFC preference by default. r=arthurcc
Bumping manifests a=b2g-bump