Bumping gaia.json for 4 gaia-central revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/b815fca222f5
Author: Jose M. Cantera <jmcanterafonseca@gmail.com>
Desc: Merge pull request #12419 from rnowm/920393

Bug 920393 - Headers [BB] wrong margin in left button

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https://hg.mozilla.org/integration/gaia-central/rev/7034960a8d53
Author: rnowm <arnau@arnaumarch.com>
Desc: Bug 920393 - Headers [BB] wrong margin in left button

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https://hg.mozilla.org/integration/gaia-central/rev/9ff0fbf48c1a
Author: Arnau <arnau@arnaumarch.com>
Desc: Merge pull request #12035 from rnowm/disabled-list

Bug 912437 - [BB] Change the disabled state for list elements row

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https://hg.mozilla.org/integration/gaia-central/rev/a4d64801cbbc
Author: rnowm <arnau@arnaumarch.com>
Desc: Bug 912437 - [BB] Change the disabled state for list elements row