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

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https://hg.mozilla.org/integration/gaia-1_4/rev/3907b745ff3d
Author: Ryan VanderMeulen <rvandermeulen@mozilla.com>
Desc: Merge pull request #18428 from justindarc/bug992393-v1.4

Bug 992393 - [camera][madai] preview mode UI updates

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https://hg.mozilla.org/integration/gaia-1_4/rev/49efa48acd83
Author: Justin D'Arcangelo <justindarc@gmail.com>
Desc: Bug 992393 - [camera][madai] preview mode UI updates
Bumping manifests a=b2g-bump