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

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https://hg.mozilla.org/integration/gaia-central/rev/6b51afcf6d39
Author: Zac <zcampbell@mozilla.com>
Desc: Merge pull request #19205 from bebef1987/test3

Bug 996252 - Intermittent TEST-UNEXPECTED-FAIL | test_settings_change

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https://hg.mozilla.org/integration/gaia-central/rev/f44fddb1a034
Author: Bebe <bebe@mozilla.ro>
Desc: Bug 996252 - Intermittent TEST-UNEXPECTED-FAIL | test_settings_change_language.py test_settings_change_language
Bumping manifests a=b2g-bump