Bumping gaia.json for 4 gaia-central revision(s)

Truncated some number of revisions since the previous bump.

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https://hg.mozilla.org/integration/gaia-central/rev/28ab56156acd
Author: Julien Wajsberg <felash@gmail.com>
Desc: Revert "Bug 906316 - Don't download the xulrunner SDK each time we change a branch if their configuration are different r=yurenju"

This reverts commit 726777cbe7dc4f3a3731fcf4629bea56c352a603.

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https://hg.mozilla.org/integration/gaia-central/rev/080732883838
Author: Julien Wajsberg <felash@gmail.com>
Desc: Revert "Bug 908698 - Gaia unit tests: /bin/bash: ./xulrunner-sdk-26/bin/run-mozilla.sh: No such file or directory r=ochameau"

This reverts commit cb1ef14cfd641ed2d6ba99402af57f28ae35dbc2.

Conflicts:
	Makefile

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https://hg.mozilla.org/integration/gaia-central/rev/059e66386680
Author: Julien Wajsberg <felash@gmail.com>
Desc: Revert "Merge pull request #11710 from lightsofapollo/fix-emails-build-system"

This reverts commit b8c30703e2898448c4ccf0032319bba16b1c5173, reversing
changes made to 14f47e5390f6fb9b0c4c841f34024870b7052cfb.

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https://hg.mozilla.org/integration/gaia-central/rev/65a55ae637d8
Author: Julien Wajsberg <felash@gmail.com>
Desc: Revert "Merge pull request #11750 from yurenju/909190"

This reverts commit adb9190fedfafa89174f32e1eb520763b7b26fc3, reversing
changes made to 43cf923c3020d77a9fa962cc0c30fd27f647660d.

b2g/config/gaia.json

@@ -1,4 +1,4 @@
 {
-    "revision": "ffe6be3d548bdcc9dba7c955e67ec4545479af50", 
+    "revision": "28ab56156acd7bf828476f31f54dd40d3a2a7cd1", 
     "repo_path": "/integration/gaia-central"
 }