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

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https://hg.mozilla.org/integration/gaia-central/rev/1a7e7c94fdc6
Author: Bob Silverberg <bob.silverberg@gmail.com>
Desc: Merge pull request #15253 from bebef1987/system_message

enable test_system_message.py Bug 956127

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https://hg.mozilla.org/integration/gaia-central/rev/66f9d005dd79
Author: Bebe <bebe@mozilla.ro>
Desc: Bug 956127 - [UITests] Travis Intermittent: test_system_message.py