Bumping gaia.json for 6 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/4e7f52475d30
Author: John Hu <johu@mozilla.com>
Desc: Merge pull request #17807 from huchengtw-moz/system2/Bug_971554-reland-2

Bug 971554 - [System2] Instantiable SoundManager, r=alive

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https://hg.mozilla.org/integration/gaia-central/rev/513eb0069324
Author: John Hu <johu@mozilla.com>
Desc: Bug 971554 - [System2] Instantiable SoundManager, r=alive

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https://hg.mozilla.org/integration/gaia-central/rev/78f618282d9b
Author: Julien Wajsberg <felash@gmail.com>
Desc: Merge pull request #17823 from julienw/987445-remove-vertical-align-middle-for-icon-buttons

Bug 987445 - [B2G][SMS]Delete button behavior intermittently unexpected ...

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https://hg.mozilla.org/integration/gaia-central/rev/b1ca52557440
Author: Julien Wajsberg <felash@gmail.com>
Desc: Bug 987445 - [B2G][SMS]Delete button behavior intermittently unexpected when no conversations are selected r=arnau

This removes the 'vertical-align: middle' property for icon buttons.

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https://hg.mozilla.org/integration/gaia-central/rev/4a10a9d9b822
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Merge pull request #17779 from evanxd/bug-984572

Bug 984572 - JSHint fixes for js/*, js/utils and js/worker.

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https://hg.mozilla.org/integration/gaia-central/rev/494e9d90d085
Author: Evan Xd <itoyxd@gmail.com>
Desc: Bug 984572 - JSHint fixes for js/*, js/utils and js/worker.
Bumping manifests a=b2g-bump