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

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https://hg.mozilla.org/integration/gaia-central/rev/f51b7f317f26
Author: Timothy Guan-tin Chien <timdream@gmail.com>
Desc: Merge pull request #21226 from timdream/keyboard-press-manager2

  Bug 1025520 - Move touch event handling away from keyboard.js, r=rudyl

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https://hg.mozilla.org/integration/gaia-central/rev/6bdd11cbe5f7
Author: Timothy Guan-tin Chien <timdream@gmail.com>
Desc: Bug 1025520 - (Part III) Let keyboard.js use UserPressManager

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https://hg.mozilla.org/integration/gaia-central/rev/2c4a85b595d2
Author: Timothy Guan-tin Chien <timdream@gmail.com>
Desc: Bug 1025520 - (Part II) Remove unused KeyboardMenuScroll dead code

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https://hg.mozilla.org/integration/gaia-central/rev/c3ea969cb238
Author: Timothy Guan-tin Chien <timdream@gmail.com>
Desc: Bug 1025520 - (Part I) Introduce UserPressManager
Bumping manifests a=b2g-bump