chore: removing engines limit

[aversini]

Fixes #29

[NickColley]

I'm okay with this if we add testing for Node.js 6 (and 4 if possible)

Based on https://github.com/nodejs/Release

I'm worried otherwise I won't notice if this package starts failing in older versions of Node.js

[NickColley]

This can be done similar to https://github.com/pouchdb/pouchdb/blob/master/.travis.yml#L107, if you don't get time to do this, I can help out.

[aversini]

Thanks ! I’ll try to find time. It’s not as simple as just updating the Travis config file right? I would also have to transpile the tests since they use async/await...

[NickColley]

I've updated the project to have a minimum of 6.x with #40, will release 3.x soon.

Thanks for raising this!