big-bug: Upgrade to npm 5.6

[andreineculau]

Both LTS and current version ship with npm 5.5.1 which has a big-bug: it's not able to respect .npmignore when packing. Npm > 5.3.0 <5.6.0 are affected.

See npm/npm#18461

[bnoordhuis]

See #17535, an upgrade to 5.6.0 is in the works.

[addaleax]

@goliatone No, this issue has long been solved.