Add ethereumjs-util as explicit dependency

This bumps the used ethereumjs-util from 4.5.0 to 6.2.0. The existing
dependency bitgo already depended on ethereumjs-util which is why an
older version was already included in the dependency tree.

Author: Sword-Smith

package-lock.json

@@ -27,6 +27,7 @@
     "eosjs": "^16.0.9",
     "eosjs-ecc": "^4.0.4",
     "ethereumjs-tx": "^1.3.7",
+    "ethereumjs-util": "6.2.0",
     "express": "^4.16.3",
     "jsrender": "^0.9.90",
     "lodash": "^4.17.11",