chore(package): update @types/chai to version 4.2.7

Closes #91

Author: greenkeeper[bot]

package.json

@@ -76,7 +76,7 @@
     "web3-utils": "^1.0.0-beta.55"
   },
   "devDependencies": {
-    "@types/chai": "4.1.7",
+    "@types/chai": "4.2.7",
     "@types/debug": "0.0.31",
     "@types/download": "6.2.3",
     "@types/express": "4.16.0",