Bumping gaia.json for 6 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/13c91c128624
Author: Luke Chang <lchang@mozilla.com>
Desc: Merge pull request #21603 from begeeben/994571_remove_the_pattern

Bug 994571 - [System2] Remove the pattern to return self in start(), r=alive

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https://hg.mozilla.org/integration/gaia-central/rev/942a2bc19dfb
Author: Yi-Fan Liao <begeeben@gmail.com>
Desc: Bug 994571 - [System2] Remove the pattern to return self in start()

* Remove 'var module = new Module().start()' pattern.
* Remove 'return this;' expression in start() function definitions.

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https://hg.mozilla.org/integration/gaia-central/rev/64e3043fb72b
Author: Pavel Ivanov <pivanov@mozilla.com>
Desc: Merge pull request #21734 from pivanov/bug-1035718

Bug 1035718 - [FTU] - remove unused images

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https://hg.mozilla.org/integration/gaia-central/rev/4f7669bf5440
Author: Pavel Ivanov <pivanov@mozilla.com>
Desc: Bug 1035718 - [FTU] - remove unused images

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https://hg.mozilla.org/integration/gaia-central/rev/03c888b37e2c
Author: Aleh Zasypkin <aleh.zasypkin@gmail.com>
Desc: Merge pull request #21776 from azasypkin/bug-1022575-on-delete-v2.0

 Bug 1022575 - Received SMS store (without text) on check balance. Listening "ondelete" event for the deleted threads. r=julien

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https://hg.mozilla.org/integration/gaia-central/rev/520280b75753
Author: Aleh Zasypkin <azasypkin@mozilla.com>
Desc: Bug 1022575 - Received SMS store (without text) on check balance. Listening "ondelete" event for the deleted threads. r=julien
Bumping manifests a=b2g-bump