Force cortex version to 0.6.2

There might have been a breaking change in 0.6.3 that prevents it from
working

package.json

@@ -11,7 +11,7 @@
   },
   "devDependencies": {
     "bootstrap": "^3.3.1",
-    "cortexjs": "^0.6.2",
+    "cortexjs": "0.6.2",
     "reqwest": "^1.1.5",
     "react": "^0.12.2",
     "react-mini-router": "^1.0.0",