chore: support node.js >=6, remove safe-buffer

[bjohansebas]

Since we are preparing version 0.7.0 (#334), this is technically a new major release, so we can remove some of the old Node.js support. I didn’t want to be too drastic since version 1 is the one planned to remove most of the legacy code and obviously the old support, to align it with Express support.

Dropping support for Node < 6 helps remove safe-buffer (Since after Node 6 this package is no longer needed to cover the purpose of safe-buffer.)(I still need to check if webpack still compiles this package properly, let’s wait and see what comes out in CI).

I could update the linter rule to already use let/const, but I’d rather do that after the release to properly verify the behavior

closes #322

cc: @blakeembrey

[coveralls]

Pull Request Test Coverage Report for Build 17057459238

Details

• 9 of 9 (100.0%) changed or added relevant lines in 8 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.006%) to 93.831%

---

Totals
Change from base Build 17015959667:	-0.006%
Covered Lines:	943
Relevant Lines:	1005

---

💛 - Coveralls

[bjohansebas]

It slightly reduces the webpack build.

Master branch:

This pr:

[blakeembrey]

Awesome, thanks!