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

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https://hg.mozilla.org/integration/gaia-central/rev/b919f497d334
Author: Sam Foster <sam@sam-i-am.com>
Desc: Merge pull request #20111 from sfoster/bug-1020435-strings

Bug 1020435 - 'Hold an' not 'Hold on an' icon in new FTU strings. r=fcampo

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https://hg.mozilla.org/integration/gaia-central/rev/c4873736ae00
Author: Sam Foster <sfoster@mozilla.com>
Desc: Bug 1020435 - One word fix to finalize FTU tutorial strings
Bumping manifests a=b2g-bump