Remove npm version check

[sonicdoe]

The current Node.js version check (which is used to determine whether to skip the npm version check) is actually the wrong way around:

np/lib/prerequisite.js

Line 93 in cec8e00

skip: () => version.isVersionLower('6.0.0', process.version),

For example, version.isVersionLower('6.0.0', 'v11.6.0') returns false, therefore not skipping the following npm version check:

np/lib/prerequisite.js

Line 97 in cec8e00

if (!version.satisfies(versions.npm, '>=2.15.8 <3.0.0 || >=3.10.1')) {

Since most installations nowadays include a newer npm version, the error was probably rarely thrown (even though the task ran on newer Node.js versions). However, Node.js v11.6.0 includes npm v6.5.0-next.0, a pre-release version. Pre-release versions do not satisfy by default which means that the above check fails and the error is thrown.

Instead of fixing the actual error, I’ve decided to remove the whole npm version check since np requires Node.js v8 by now (and won’t actually run on Node.js v6).

Fixes #309.