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

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https://hg.mozilla.org/integration/gaia-1_4/rev/b2894d18cc3b
Author: dwi2 <dwi2@dwi2.com>
Desc: Merge pull request #18116 from dwi2/bug975293

Bug 975293 - Make sure we have _containerElement defined in the context ...

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https://hg.mozilla.org/integration/gaia-1_4/rev/76e169da425f
Author: James Burke <jrburke@gmail.com>
Desc: Merge pull request #17561 from jrburke/bug985598-email-double-activity-blank

Bug 985598 - [email] Double-triggering of share/new activity by clicking... r=asuth
Bumping manifests a=b2g-bump