Bumping gaia.json for 2 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/6bc990a70f2f
Author: gasolin <gasolin@gmail.com>
Desc: Merge pull request #16192 from gasolin/issue-971397

Bug 971397 - [customization] usage guide referenced in README is moved t..., r=yurenju

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https://hg.mozilla.org/integration/gaia-central/rev/b237fdefb2d2
Author: gasolin <gasolin@gmail.com>
Desc: Bug 971397 - [customization] usage guide referenced in README is moved to MDN

b2g/config/gaia.json

@@ -1,4 +1,4 @@
 {
-    "revision": "19aba7aaa44231efa54aed826d3024b1118082d4", 
+    "revision": "6bc990a70f2f506b0ad44ef23aff268c834fb729", 
     "repo_path": "/integration/gaia-central"
 }