Update Node.js version for Travis

[jfmengels]

Update Node.js version for Travis

[jfmengels]

Not that surprising, but it works on v6, yay :)

[ntwb]

Should this change 5 to 6 rather than just adding 6?

It is really just a major semver version bump than a new release branch IMHO.

See also: NodeJS as part of 6.0.0 have renamed stable to current in nodejs/node/pull/6318

[sindresorhus]

Yes, let's drop testing on Node.js 5. We're only interested in testing on current and LTS releases.

Node.js v5 will continue to be supported for the next two months in order to give developers currently using v5 time to transition to Node.js v6. - https://nodejs.org/en/blog/release/v6.0.0/

[novemberborn]

@sindresorhus while I doubt there'd be regressions that affect just v5, it seems sensible to keep testing until official support is withdrawn. Especially in the first few weeks when people won't necessarily have upgraded.

It is really just a major semver version bump than a new release branch IMHO.

@ntwb it's a major bump because of the breaking changes, which weren't previously in v5.