Upgrade dev dependency istanbul

Author: rjmunro

package.json

@@ -49,7 +49,7 @@
   ],
   "devDependencies": {
     "chai": "4.1.2",
-    "istanbul": "0.3.17",
+    "istanbul": "0.4.5",
     "jscs": "1.13.1",
     "jshint": "2.8.0",
     "mocha": "2.2.5"