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

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https://hg.mozilla.org/integration/gaia-central/rev/6c1612717329
Author: Evyatar Amitay <evyatron@gmail.com>
Desc: Merge pull request #14316 from EverythingMe/911778-cloud-apps-to-homescreen

[Bug 911778] When saving apps to homescreen - use correct size [r=ranbena]

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https://hg.mozilla.org/integration/gaia-central/rev/b1332108920b
Author: Evyatar Amitay <evyatron@gmail.com>
Desc: [Bug 911778] When saving apps to homescreen - use correct size [r=ranbena]

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https://hg.mozilla.org/integration/gaia-central/rev/46f5ed3178a1
Author: Jose M. Cantera <jmcanterafonseca@gmail.com>
Desc: Merge pull request #14389 from jmcanterafonseca/fix_ds_read_revisionid

Bug 946600 - When datastore is open in read-only mode revisionId is not ...

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https://hg.mozilla.org/integration/gaia-central/rev/28a3bc6a6db6
Author: Jose M. Cantera <jmcanterafonseca@gmail.com>
Desc: Bug 946600 - When datastore is open in read-only mode revisionId is not properly obtained