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

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https://hg.mozilla.org/integration/gaia-central/rev/175bcf94c3ff
Author: Yuren Ju <yurenju@gmail.com>
Desc: Merge pull request #12040 from yurenju/test-agent-vbump

Bug 825913 - vbump for test-agent r=NPOTB

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https://hg.mozilla.org/integration/gaia-central/rev/31dd51b582a4
Author: Yuren Ju <yurenju@gmail.com>
Desc: Bug 914234 - vbump for test-agent

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https://hg.mozilla.org/integration/gaia-central/rev/1aab48312ec2
Author: gasolin <gasolin@gmail.com>
Desc: Merge pull request #12053 from gasolin/issue-914477

Bug 914477 - fix regression of permission manager, r=alive

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https://hg.mozilla.org/integration/gaia-central/rev/fe72e31016f8
Author: gasolin <gasolin@gmail.com>
Desc: Bug 914477 - fix regression of permission manager

b2g/config/gaia.json

@@ -1,4 +1,4 @@
 {
-    "revision": "38e13b287b956f7e41c3ce591b5bc49dc29b6c80", 
+    "revision": "175bcf94c3ff3aab3e5d0bdaf05427602f658406", 
     "repo_path": "/integration/gaia-central"
 }