Bumping gaia.json for 2 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/53c3656a9e95
Author: gasolin <gasolin@gmail.com>
Desc: Merge pull request #18448 from gasolin/issue-968695

Bug 968695 - [settings] add marionette test for Application storage pane..., r=arthur

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https://hg.mozilla.org/integration/gaia-central/rev/15d58dfb8ee1
Author: gasolin <gasolin@gmail.com>
Desc: Bug 968695 - [settings] add marionette test for Application storage panel
Bumping manifests a=b2g-bump