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

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https://hg.mozilla.org/integration/gaia-1_4/rev/dc491bc4fe6d
Author: David Flanagan <dflanagan@mozilla.com>
Desc: Merge pull request #18251 from davidflanagan/bug993512-v1.4

Merge pull request #18246 from justindarc/bug993512

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https://hg.mozilla.org/integration/gaia-1_4/rev/f40fdf61b1e4
Author: David Flanagan <dflanagan@mozilla.com>
Desc: Merge pull request #18246 from justindarc/bug993512

Bug 993512 - [B2G][Camera] Retaking a photo will briefly display previou... r=djf(cherry picked from commit 4ffb7c8cb330fc22cba938223443f056c7bc03b8)
Bumping manifests a=b2g-bump