Bumping gaia.json for 1 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/5a91bb3fa3d8
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Bug 1018393 - Disable intermittent failing test, failing test, test_a11y_keyboard_key_names.py test_a11y_keyboard_key_names.TestA11yKeyboardKeyNames.test_a11y_keyboard_key_names
Bumping manifests a=b2g-bump