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

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https://hg.mozilla.org/integration/gaia-central/rev/50dfc898b9dc
Author: RickyChien <ricky060709@gmail.com>
Desc: Merge pull request #18816 from RickyChien/bug-988259

Bug 988259 - [TestAgent] The "small screen" css is broken r=@julienw

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https://hg.mozilla.org/integration/gaia-central/rev/b7c396fc57f0
Author: Ricky Chien <ricky060709@gmail.com>
Desc: Bug 988259 - [TestAgent] The "small screen" css is broken
Bumping manifests a=b2g-bump