Prevent fingerprinting for filenames from config

Ember has no way of finding them in the code and thus cannot turn hrefs
into fingerprinted URLs

ember-cli-build.js

@@ -29,6 +29,9 @@ module.exports = function(defaults) {
           path: "empty" // needed for kosmos-schemas dependency
         }
       }
+    },
+    fingerprint: {
+      exclude: [ 'img/icon-btc' ]
     }
   });