Upgrade dev dependency chai

Author: rjmunro

package.json

@@ -48,7 +48,7 @@
     "lib"
   ],
   "devDependencies": {
-    "chai": "2.3.0",
+    "chai": "4.1.2",
     "istanbul": "0.3.17",
     "jscs": "1.13.1",
     "jshint": "2.8.0",