Updated ethereumjs-tx dev dependency from 1.3.3 to 1.3.7

Author: holgerd77

package.json

@@ -53,7 +53,7 @@
     "coveralls": "^3.0.0",
     "ethereumjs-blockchain": "~3.3.0",
     "ethereumjs-testing": "git+https://github.com/ethereumjs/ethereumjs-testing.git#v1.2.4",
-    "ethereumjs-tx": "1.3.3",
+    "ethereumjs-tx": "1.3.7",
     "level": "^1.4.0",
     "leveldown": "^1.4.6",
     "levelup": "^1.3.2",