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

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https://hg.mozilla.org/integration/gaia-2_1/rev/2efcc2e98c18
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Bug 1061679 - [Browser Chrome] Press states of back, refresh, cancel and overflow menu using the wrong icons r=epang

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https://hg.mozilla.org/integration/gaia-2_1/rev/d258af4565f3
Author: Eli Perelman <eli@eliperelman.com>
Desc: Fixing SHB visibility in app landscape mode r=kgrandon

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https://hg.mozilla.org/integration/gaia-2_1/rev/c1c5d404a87b
Author: Eli Perelman <eli@eliperelman.com>
Desc: Bug 1067441 - Enable SHB on secured lockscreen apps

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https://hg.mozilla.org/integration/gaia-2_1/rev/49987d4096dc
Author: Timothy Guan-tin Chien <timdream@gmail.com>
Desc: Bug 1066654 - Set aria-pressed on Shift key
Bumping manifests a=b2g-bump