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

Author: CvX

.travis.yml

@@ -8,6 +8,8 @@ branches:
 
 cache:
   cargo: true
+  directories:
+    - $HOME/.npm
 
 env:
   global: