Strict mode error on Travis with Node.js 4 or 5

[jogold]

Hello,

When running greenkeeper-lockfile-upload on Travis with Node.js 4 or 5, I'm getting a strict mode error. Shouldn't 'use strict' be added to update.js and upload.js?

/home/travis/.nvm/versions/node/v5.12.0/lib/node_modules/greenkeeper-lockfile/upload.js:36
  let remote = `[email protected]:${info.repoSlug}`
  ^^^
SyntaxError: Block-scoped declarations (let, const, function, class) not yet supported outside strict mode
    at exports.runInThisContext (vm.js:53:16)
    at Module._compile (module.js:387:25)
    at Object.Module._extensions..js (module.js:422:10)
    at Module.load (module.js:357:32)
    at Function.Module._load (module.js:314:12)
    at Function.Module.runMain (module.js:447:10)
    at startup (node.js:148:18)
    at node.js:405:3

[jogold]

The error can been seen here also: https://travis-ci.org/greenkeeperio/greenkeeper-lockfile/jobs/242089225