Bumping gaia.json for 4 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/6a497c0f6c46
Author: Mason Chang <mason@masonchang.com>
Desc: Merge pull request #15617 from changm/bug962699

Bug 962699 Put Email App in Scrollable Background Layer. r=jrburke

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https://hg.mozilla.org/integration/gaia-central/rev/bd17f7ea7172
Author: Mason Chang <mchang@mozilla.com>
Desc: Bug 962699 Put Email App in Scrollable Background Layer

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https://hg.mozilla.org/integration/gaia-central/rev/77dcf510d7f4
Author: George <georgeiscoming@gmail.com>
Desc: Merge pull request #14958 from cctuan/921828

Bug 921828 - (app-menu) [Window Management] Refactor ListMenu and move it to AppWindow + implement System Wide Menu if needed

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https://hg.mozilla.org/integration/gaia-central/rev/ebe0bab7a7ea
Author: cctuan <georgeiscoming@gmail.com>
Desc: Bug 921828 - (app-menu) [Window Management] Refactor ListMenu and move it to AppWindow + implement System Wide Menu if needed
Bumping manifests a=b2g-bump