chore(package): update @types/chai to version 4.2.8

Closes #154

Author: greenkeeper[bot]

package.json

@@ -83,7 +83,7 @@
         "test": "gulp test"
     },
     "devDependencies": {
-        "@types/chai": "4.1.7",
+        "@types/chai": "4.2.8",
         "@types/mocha": "5.2.7",
         "@types/node": "12.6.7",
         "chai": "4.1.2",