Build/Test tools: Switch npm dependency caching strategy on Travis CI.

This switches to caching npm's local cache instead of `node_modules` in order to prevent issues caused by modules compiled using a different version of node.

See #46632


git-svn-id: https://develop.svn.wordpress.org/trunk@44993 602fd350-edb4-49c9-b593-d223f7449a82

.travis.yml

@@ -4,7 +4,7 @@ language: php
 cache:
   apt: true
   directories:
-    - node_modules
+    - $HOME/.npm
     - vendor
     - $HOME/.composer/cache
 env: