chore(package): update chai to version 4.0.2

Author: greenkeeper[bot]

package.json

@@ -32,7 +32,7 @@
   },
   "homepage": "https://github.com/flowbased/fbp-diff#readme",
   "devDependencies": {
-    "chai": "^3.5.0",
+    "chai": "^4.0.2",
     "coffee-script": "^1.10.0",
     "js-yaml": "~3.8.3",
     "fbp": "^1.5.0",