chore(package): update dependencies

https://greenkeeper.io/

package.json

@@ -24,18 +24,18 @@
   },
   "homepage": "https://github.com/ethereumjs/ethereumjs-blockchain#readme",
   "dependencies": {
-    "async": "^1.5.0",
+    "async": "^2.1.2",
     "ethashjs": "^0.0.7",
     "ethereumjs-block": "^1.2.2",
-    "ethereumjs-util": "^4.0.1",
+    "ethereumjs-util": "^5.0.1",
     "levelup": "^1.3.0",
     "memdown": "^1.1.0",
     "semaphore": "^1.0.3"
   },
   "devDependencies": {
     "coveralls": "^2.11.4",
     "istanbul": "^0.4.2",
-    "standard": "^5.4.1",
+    "standard": "^8.5.0",
     "tape": "^4.2.1"
   }
 }