Bumping gaia.json for 3 gaia-central revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/319623e1caef
Author: Zac Campbell <zcampbell@mozilla.com>
Desc: Revert "Bug 928885 - xfail test_import_contacts_from_sim because"

This reverts commit b3b08a024fafb93ab25f8b8501cc407ddf18a35e.

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https://hg.mozilla.org/integration/gaia-central/rev/aee827d584d4
Author: Fernando Campo <fernando.campo@o2.com>
Desc: Merge pull request #12943 from gitmai/bug-928499-no-background-image

Bug 928499 - Background image (r=fcampo)

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https://hg.mozilla.org/integration/gaia-central/rev/8ffca287f95c
Author: mai <mri@tid.es>
Desc: Bug 928499 - Background image

b2g/config/gaia.json

@@ -1,4 +1,4 @@
 {
-    "revision": "44222c9172315825198b95ee3f7df7ffc71f1b65", 
+    "revision": "319623e1caefed1d283cdeeda902f9a436f0d4b3", 
     "repo_path": "/integration/gaia-central"
 }