Fails on Node 4

[pvdlg]

I have a package-lock.json but no npm-shrinkwrap.json nor yarn.lock.
On Node 4 it fails with the following error:

/home/travis/.nvm/versions/node/v4.8.4/lib/node_modules/greenkeeper-lockfile/lib/update-lockfile.js:39
      const flag = flags[dependency.type]
      ^
SyntaxError: Identifier 'flag' has already been declared
    at updateLockfile (/home/travis/.nvm/versions/node/v4.8.4/lib/node_modules/greenkeeper-lockfile/lib/update-lockfile.js:23:42)
    at Module.update [as exports] (/home/travis/.nvm/versions/node/v4.8.4/lib/node_modules/greenkeeper-lockfile/update.js:60:3)
    at Object.<anonymous> (/home/travis/.nvm/versions/node/v4.8.4/lib/node_modules/greenkeeper-lockfile/update.js:68:37)
    at Module._compile (module.js:409:26)
    at Object.Module._extensions..js (module.js:416:10)
    at Module.load (module.js:343:32)
    at Function.Module._load (module.js:300:12)
    at Function.Module.runMain (module.js:441:10)
    at startup (node.js:140:18)
    at node.js:1043:3

Here is an example: https://travis-ci.org/vanduynslagerp/karma-rollup-preprocessor/jobs/263515447

It works fine on Node 6 and up (I didn't test for Node 5).