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

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https://hg.mozilla.org/integration/gaia-central/rev/88b550f80d8d
Author: Jed Parsons <jedp@me.com>
Desc: Merge pull request #17105 from jedp/929917-uitest-fxa

Bug 929917 - UI Test app for Firefox Accounts DOM API

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https://hg.mozilla.org/integration/gaia-central/rev/31c35976b870
Author: Jed Parsons <jedp@me.com>
Desc: Bug 929917 - UI Test app for Firefox Accounts DOM API
Bumping manifests a=b2g-bump