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

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https://hg.mozilla.org/integration/gaia-central/rev/b62ac34d65ce
Author: Zac <zcampbell@mozilla.com>
Desc: Merge pull request #12917 from zacc/bug_927021

Bug 927021 - Re-enable python UI test test_system_notification_bar.py

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https://hg.mozilla.org/integration/gaia-central/rev/05749e1425ac
Author: Zac Campbell <zcampbell@mozilla.com>
Desc: Bug 927021 - Re-enable python UI test test_system_notification_bar.py