63 update engine

[ricellis]

Overview

Updated the minimum Node.js engine in package.json to 6. This is currently the oldest available LTS release of Node.js. Moving to a Node.js version >4 unblocks upgrades of request to versions > 2.81.0 so the workaround from fc794b2 to pin to 2.81.0 can be safely removed (although it appears that #73 already moved it to 2.83.0). With Node.js 6 it should be safe to move back to backwards compatible changes.

Testing recommendations

npm install will give a warning on Node.js versions older than 6.

GitHub issue number

#63

Checklist

• Code is written and works correctly; - No new code package.json and request dependency version change only.
• Changes are covered by tests; - tests pass locally for me.
• Documentation reflects the changes; - updated README to mention the minimum version.