Bumping gaia.json for 2 gaia-central revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/1559f4573bc0
Author: Michael Henretty <michael.henretty@gmail.com>
Desc: Merge pull request #15272 from mikehenrty/bug-952761-lockscreeny

Bug 952761 lockscreeny, r=gaye

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https://hg.mozilla.org/integration/gaia-central/rev/bd01805352c7
Author: Michael Henretty <michael.henretty@gmail.com>
Desc: Bug 952761 - re-enable lock screen test