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

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https://hg.mozilla.org/integration/gaia-central/rev/51bb0dde2b98
Author: Zac <zcampbell@mozilla.com>
Desc: Merge pull request #23951 from bebef1987/email_fix

Bug 1065930 - Email test_setup_* failing due to HTML changes

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https://hg.mozilla.org/integration/gaia-central/rev/4e120ec1ae4f
Author: Bebe <bebe@mozilla.ro>
Desc: Bug 1065930 - Email test_setup_* failing due to HTML changes
Bumping manifests a=b2g-bump