[enh] Upgrade Duniter devDependency to 1.6.x

package.json

@@ -54,7 +54,7 @@
     "brunch": "2.10.9",
     "core-util-is": "1.0.2",
     "css-brunch": "2.0.0",
-    "duniter": "1.4.x",
+    "duniter": "1.6.x",
     "fb-flo-brunch": "1.7.22",
     "jade-brunch": "2.0.0",
     "javascript-brunch": "2.0.0",