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

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https://hg.mozilla.org/integration/gaia-central/rev/d57a6f654752
Author: Zac <zcampbell@mozilla.com>
Desc: Merge pull request #13141 from zacc/bug_931793

Bug 931793 - fix incorrect is_displayed() usage

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https://hg.mozilla.org/integration/gaia-central/rev/4a83b4c6c186
Author: Zac Campbell <zcampbell@mozilla.com>
Desc: Bug 931793 - fix incorrect is_displayed() usage