chore(package): update @types/node-fetch to version 2.5.7

Closes #385

Author: greenkeeper[bot]

package.json

@@ -56,7 +56,7 @@
     "@types/ethereumjs-tx": "1.0.1",
     "@types/lokijs": "1.5.2",
     "@types/node": "11.9.4",
-    "@types/node-fetch": "2.1.6",
+    "@types/node-fetch": "2.5.7",
     "@types/web3": "1.0.18",
     "chai": "4.2.0",
     "coveralls": "3.0.2",