chore(package): update @types/chai to version 4.2.6

Closes #34

Author: greenkeeper[bot]

package.json

@@ -31,7 +31,7 @@
     "typescript": "^3.3.3333"
   },
   "devDependencies": {
-    "@types/chai": "4.1.4",
+    "@types/chai": "4.2.6",
     "@types/lodash": "4.14.131",
     "@types/mocha": "5.2.6",
     "chai": "4.1.2",