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

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https://hg.mozilla.org/integration/gaia-central/rev/cbab228dbf95
Author: Al Tsai <atsai@mozilla.com>
Desc: Merge pull request #15710 from altsai/bug948826-2

bug 948826: privileged app prompt testing

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https://hg.mozilla.org/integration/gaia-central/rev/0b18bd21eafc
Author: Al Tsai[:atsai] <atsai@mozilla.com>
Desc: bug 948826: privileged app prompt testing

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https://hg.mozilla.org/integration/gaia-central/rev/c910018457fc
Author: James Burke <jrburke@gmail.com>
Desc: Merge pull request #15739 from jrburke/bug962451-email-empty-list

Bug 962451 - [email] Start from activity, tap back from compose/message reader, empty message list r=asuth

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https://hg.mozilla.org/integration/gaia-central/rev/45eb6cd8be27
Author: jrburke <jrburke@gmail.com>
Desc: Bug 962451 - [email] Start from activity, tap back from compose/message reader, empty message list
Bumping manifests a=b2g-bump