Update dependencies of benchmarks (#637)

Author: morolt

test/benchmarks/package.json

@@ -5,12 +5,12 @@
   "main": "index.js",
   "private": true,
   "dependencies": {
-    "async": "~0.1.22",
-    "benchmark": "~1.0.0",
+    "async": "^2.6.1",
+    "benchmark": "^2.1.4",
     "colors": "~0.6.0-1",
-    "leveldown": "^1.7.2",
+    "leveldown": "^4.0.1",
     "levelup": "*",
-    "rimraf": "~2.0.2",
+    "rimraf": "^2.6.3",
     "sqlite3": "*"
   }
 }