Add diff dependency required by karma-mocha-reporter when the showDiff option is enabled

package.json

@@ -70,6 +70,7 @@
   "devDependencies": {
     "chai": "^3.5.0",
     "check-dependencies": "^0.12.0",
+    "diff": "^2.2.2",
     "jscs": "^3.0.2",
     "karma": "^0.13.22",
     "karma-browserify": "^5.0.3",