Update dependency @types/chai to v4.2.8

Author: renovate-bot

package.json

@@ -33,7 +33,7 @@
   },
   "devDependencies": {
     "@istanbuljs/nyc-config-typescript": "0.1.3",
-    "@types/chai": "4.1.7",
+    "@types/chai": "4.2.8",
     "@types/chai-as-promised": "7.1.0",
     "@types/mocha": "5.2.7",
     "@types/sinon": "7.0.13",