chore(build): add missing karma-coverage dependency

Useful for retrieving test coverage statistics.

Author: chrisirhc

Diff for: package.json

@@ -24,6 +24,7 @@
     "grunt-ngdocs": "~0.1.1",
     "karma": "^0.12.31",
     "karma-chrome-launcher": "^0.1.7",
+    "karma-coverage": "^0.2.7",
     "karma-firefox-launcher": "^0.1.4",
     "karma-jasmine": "^0.1.5",
     "node-markdown": "0.1.1",