Drop support for end-of-life Node versions

[Evan Hahn]

Helmet currently supports Node 12, but that version is no longer supported by the Node team.

In the next major version of Helmet (and not sooner!), we should drop support for old versions.

See the Node.js release schedule for more details.

[Evan Hahn]

Done in the v6 branch. See #370 for more details.