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

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https://hg.mozilla.org/integration/gaia-central/rev/7615f637d227
Author: Andrew Sutherland <asutherland@asutherland.org>
Desc: Merge pull request #10407 from asutherland/email-progress-spinner

Bug 882994 - [email] UI: Replace sync progress bar (including candy bar) / messages spinner with single spinning refresh button. r=jrburke

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https://hg.mozilla.org/integration/gaia-central/rev/675a907c30fe
Author: Andrew Sutherland <asutherland@asutherland.org>
Desc: Bug 882994 - [email] UI: Replace sync progress bar (including candy bar) / messages spinner with single spinning refresh button. r=jrburke

This is both a performance win (animations chew CPU, especially the candybar)
and a UX win since it gets rid of the confusing determinate progress bar
status.