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

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

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https://hg.mozilla.org/integration/gaia-2_1/rev/c91eca38ade0
Author: Ryan VanderMeulen <ryanvm@gmail.com>
Desc: Merge pull request #27878 from cctuan/1124660

Bug 1124660 - Settings app is truncated when accessed via rocketbar "no internet connection"

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https://hg.mozilla.org/integration/gaia-2_1/rev/90e424ca6e5b
Author: cctuan <georgeiscoming@gmail.com>
Desc: Bug 1124660 - activitywindow with both inline and isActivity and launched by rocketbar should cover statusbar

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https://hg.mozilla.org/integration/gaia-2_1/rev/5bd77316af2a
Author: Ryan VanderMeulen <ryanvm@gmail.com>
Desc: Merge pull request #25796 from mancas/bug1091483_v2.1

Bug 1091483 - Unable to show stk dialog (e.g. user confirmation dialog) ...

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https://hg.mozilla.org/integration/gaia-2_1/rev/275f862414a5
Author: Manuel Casas <manuel.casasbarrado@telefonica.com>
Desc: Bug 1091483 - Unable to show stk dialog (e.g. user confirmation dialog) when foreground app is Callscreen