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

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https://hg.mozilla.org/integration/gaia-central/rev/e975c8069c2e
Author: russnicoletti <rnicoletti@mozilla.com>
Desc: Merge pull request #21635 from russnicoletti/handleSliderTouch_unit_tests

handleSliderTouchStart/Move/End unit tests r=johu

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https://hg.mozilla.org/integration/gaia-central/rev/2df26481f7e1
Author: Russ Nicoletti <rnicoletti@mozilla.com>
Desc: handleSliderTouchStart/Move/End unit tests

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https://hg.mozilla.org/integration/gaia-central/rev/e5ea863991c2
Author: Dominic Kuo <dkuo@mozilla.com>
Desc: Merge pull request #21692 from dominickuo/bug-1032638

Bug 1032638 - [Music] Ignore the MP4 files with not enough metadata, r=jimporter

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https://hg.mozilla.org/integration/gaia-central/rev/a215c770793a
Author: Dominic Kuo <dkuo@mozilla.com>
Desc: Bug 1032638 - [Music] Ignore the MP4 files with not enough metadata
Bumping manifests a=b2g-bump