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

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https://hg.mozilla.org/integration/gaia-central/rev/cb2d1314690a
Author: Yuren Ju <yurenju@gmail.com>
Desc: Merge pull request #19053 from yurenju/twitter

Bug 1007561 - Using local http server to test distribution mechanism

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https://hg.mozilla.org/integration/gaia-central/rev/6adb407a1ef1
Author: Yuren Ju <yurenju@gmail.com>
Desc: Bug 1007561 - Using local http server to test distribution mechanism

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https://hg.mozilla.org/integration/gaia-central/rev/6c6e72a5bd65
Author: John Hu <johu@mozilla.com>
Desc: Merge pull request #18932 from huchengtw-moz/Stingray/Bug_1002339-layout-editor

Bug 1002339 - [Stingray] layout editor for stingray, r=garychen, r=lchang

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https://hg.mozilla.org/integration/gaia-central/rev/a9172b1b6e96
Author: John Hu <johu@mozilla.com>
Desc: Bug 1002339 - [Stingray] layout editor for stingray
Bumping manifests a=b2g-bump