Bumping gaia.json for 1 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-1_4/rev/7ab87934eae6
Author: Jim Porter <jporter@mozilla.com>
Desc: Merge pull request #18602 from jimporter/music-update-hidden-scrubber

Bug 1000214 - [music] Update progress bar when display is suspended and ...
Conflicts:
	apps/music/test/marionette/Player_test.js
Bumping manifests a=b2g-bump