Travis CI: Allow to cache the node_modules directory.

This should speed up the installation of npm dependencies.

Props netweb.
Fixes #36291.

git-svn-id: https://develop.svn.wordpress.org/trunk@37058 602fd350-edb4-49c9-b593-d223f7449a82

.travis.yml

@@ -1,5 +1,8 @@
 sudo: false
 language: php
+cache:
+  directories:
+    - node_modules
 matrix:
   include:
   - php: 5.5