Bumping gaia.json for 2 gaia-central revision(s)

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https://hg.mozilla.org/integration/gaia-central/rev/e4e28ac6f7a1
Author: Yuren Ju <yurenju@gmail.com>
Desc: Merge pull request #11882 from yurenju/911659

Bug 911659 - build script settings.js would return error when setting GA... r=@timdream

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https://hg.mozilla.org/integration/gaia-central/rev/a03631441769
Author: Yuren Ju <yurenju@gmail.com>
Desc: Bug 911659 - build script settings.js would return error when setting GAIA_DEV_PIXELS_PER_PX=1.5

b2g/config/gaia.json

@@ -1,4 +1,4 @@
 {
-    "revision": "92146439c28b97f7cc87689d74f3547d35ae16ea", 
+    "revision": "e4e28ac6f7a13ee205cdcdede3ce13aff9227997", 
     "repo_path": "/integration/gaia-central"
 }