[BREAKING] Drop support for Node <6.6.0

[wojpawlik]

Makes the package tiny and dependency-free by removing ponyfills needed only by EOL Node.js.

#13 should be released first as 1.1.5.

[coveralls]

Coverage remained the same at 100.0% when pulling bc98a8c on wojpawlik:drop-old-node into ad212fc on Bruce17:master.

[Bruce17]

I have one doubt with only using the native timing method: performance. The benchmark tests return a bad performance for the native method using Buffer.alloc and a even worse performance using Buffer.allocUnsafe.

Therefore I have to validate the benchmark itself first and then decide on how to continue here.

Any comments on that @Drexonom ?

[wojpawlik]

How can I see the benchmark results? npm run posttest outputs nothing.

[Bruce17]

Matcha in Posttest ist broken for newer Node.js versions. I just tested it with some different performance testing libraries. Maybe I can upload an example soon.