Update Travis CI Cache (#5575)

`prepare` causes `npm ci` to be used. `.npm` is the recommended cache directory. Should speed up build times.

https://docs.npmjs.com/cli/ci

Author: dplewis

.travis.yml

@@ -18,7 +18,7 @@ branches:
   - /^greenkeeper/.*$/
 cache:
   directories:
-  - node_modules
+  - "$HOME/.npm"
   - "$HOME/.mongodb/versions"
 
 # Test stage