Bumping gaia.json for 4 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/2b31a4ef1b5f
Author: autolander <bug.autolander@gmail.com>
Desc: Bug 1119190 - merge pull request #27520 from jmcanterafonseca:icon_mms_vcard to mozilla-b2g:master

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https://hg.mozilla.org/integration/gaia-central/rev/a8ccd9bcc090
Author: Jose M. Cantera <jmcanterafonseca@gmail.com>
Desc: Bug 1119190 - [MMS] Icon for enclosed contacts

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https://hg.mozilla.org/integration/gaia-central/rev/f9d63c9bc43c
Author: autolander <bug.autolander@gmail.com>
Desc: Bug 1124398 - merge pull request #28453 from etiennesegonzac:bug-1124398 to mozilla-b2g:master

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https://hg.mozilla.org/integration/gaia-central/rev/de4f6c834e8b
Author: ben.song <ben.song@spreadtrum.com>
Desc: Bug 1124398 - Edge Swipe transitioning between apps will sometimes encounter a black screen when settings app is open r=etienne

b2g/config/gaia.json

@@ -1,9 +1,9 @@
 {
     "git": {
-        "git_revision": "9595dac56dc47f458a8567d8db099900d574e914", 
+        "git_revision": "73992beebbf48c4721550f1c9c14315be1148ef3", 
         "remote": "https://git.mozilla.org/releases/gaia.git", 
         "branch": ""
     }, 
-    "revision": "fa5e356fd3c0c146ee4fdd9d9ed0e2ae343b6323", 
+    "revision": "2b31a4ef1b5f2e9035b957608e1f656e69230e05", 
     "repo_path": "integration/gaia-central"
 }