Bump ethereumjs-tx from 1.3.3 to 1.3.7 (#7)

Bumps [ethereumjs-tx](https://github.com/ethereumjs/ethereumjs-tx) from 1.3.3 to 1.3.7.
- [Release notes](https://github.com/ethereumjs/ethereumjs-tx/releases)
- [Changelog](https://github.com/ethereumjs/ethereumjs-tx/blob/master/CHANGELOG.md)
- [Commits](ethereumjs/ethereumjs-tx@v1.3.3...v1.3.7)

Signed-off-by: dependabot[bot] <support@dependabot.com>

yarn.lock

@@ -1396,8 +1396,8 @@ ethereum-common@^0.0.18:
   resolved "https://registry.yarnpkg.com/ethereum-common/-/ethereum-common-0.0.18.tgz#2fdc3576f232903358976eb39da783213ff9523f"
 
 ethereumjs-tx@^1.2.5:
-  version "1.3.3"
-  resolved "https://registry.yarnpkg.com/ethereumjs-tx/-/ethereumjs-tx-1.3.3.tgz#ece051d3efdbe771ad2a518d61632ca2ab75ecbb"
+  version "1.3.7"
+  resolved "https://registry.yarnpkg.com/ethereumjs-tx/-/ethereumjs-tx-1.3.7.tgz#88323a2d875b10549b8347e09f4862b546f3d89a"
   dependencies:
     ethereum-common "^0.0.18"
     ethereumjs-util "^5.0.0"