Cache npm packages.
Cache npm packages.

See: https://github.com/ember-cli/ember-new-output/blob/v3.0.2/.travis.yml#L14 and https://stackoverflow.com/questions/42521884/should-i-have-travis-cache-node-modules-or-home-npm

rust-lang/crates.io,rust-lang/crates.io,rust-lang/crates.io,rust-lang/crates.io