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

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https://hg.mozilla.org/integration/gaia-central/rev/6256a30275b9
Author: Rex KM Lee <rexboy@mozilla.com>
Desc: Merge pull request #10878 from rexboy7/fix-bug891408

Bug 891408 - [System] Calculate inline app size correctly (regression)

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https://hg.mozilla.org/integration/gaia-central/rev/3ad1b19036ee
Author: Rex Lee <rexboy@mozilla.com>
Desc: Bug 891408 - [System] Calculate inline app size correctly (regression)