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

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https://hg.mozilla.org/integration/gaia-central/rev/ebc87df16535
Author: Gareth Aye <gaye@mozilla.com>
Desc: Merge pull request #14544 from gaye/non-native-marionette

Use non-native marionette-js-runner for buildbot

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https://hg.mozilla.org/integration/gaia-central/rev/50345a17dc8d
Author: Gareth Aye <gaye@mozilla.com>
Desc: Use non-native marionette-js-runner for buildbot

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https://hg.mozilla.org/integration/gaia-central/rev/6f750c8ddf5b
Author: Zac <zcampbell@mozilla.com>
Desc: Merge pull request #14015 from viorelaioia/bug_942220

Bug 942220 - Add explicit wait in tap_back in contact_details.py

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https://hg.mozilla.org/integration/gaia-central/rev/a5b09f705e4f
Author: Viorela Ioia <viorelaioia@gmail.com>
Desc: Bug 942220 - Add explicit wait in tap_back in contact_details.py