Compatibility with node ^7.0

[BartWaardenburg]

When running this module with node ^7.0 yarn gives the following warning (npm ignores it and installs succesfully):

error trace@2.3.3: The engine "node" is incompatible with this module. Expected version "^4.5 || ^6.0".

[AndreasMadsen]

We are experiencing some issue in node.js, you can follow our progress at nodejs/node#9467

If you are lucky trace will still work, but it's not something that I can guarantee.

[AndreasMadsen]

Issue node.js issue (nodejs/node#9467) is now fixed, I have updated the engine requirements accordingly.