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

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https://hg.mozilla.org/integration/gaia-central/rev/7e7fe9513146
Author: Andrew Sutherland <asutherland@asutherland.org>
Desc: Merge pull request #15327 from asutherland/mail-slice-batching

Bug 862870 - [email] Implement slice batching to reduce/minimize UI reflows, improve responsiveness, sync time. r=asuth

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https://hg.mozilla.org/integration/gaia-central/rev/cadc3e8774f3
Author: Andrew Sutherland <asutherland@asutherland.org>
Desc: Bug 862870 - [email] Implement slice batching to reduce/minimize UI reflows, improve responsiveness, sync time. r=asuth

Land https://github.com/mozilla-b2g/gaia-email-libs-and-more/pull/280