BREAKING CHANGE(core): bump core deps, drop node 6 support

[xavdid]

Actual user impact here should be minimal, but should be noted. sparknotes:

• response.text() will assume and always use utf-8 encoding. Use response.textConverted() if you want to retain the old behavior (where the library would try and guess)
• Some internal methods were hidden. If you're not calling response._SOMETHING, then you're fine
• behavior around comma-separated headers changed. This is the most likely source of a user issue.

read more about the changes in fetch.

[eliangcs]

Looks good and thanks for the clear TLDR! So those are breaking changes because of node-fetch. How about Node 6 support? We're dropping Node 6 support because of a certain package upgrade, I presume?

[xavdid]

I think the only thing that is actually unsupported is dotenv, but given core won't be run in node 6 anymore anyway, I think it's mostly ceremonial. ¯\_(ツ)_/¯