Build/Test Tools: Allow Travis CI to cache the node_modules directory.

Merge of [37058] and [36490] to the 4.3 branch.

See #36291, #36490.

git-svn-id: https://develop.svn.wordpress.org/branches/4.3@40281 602fd350-edb4-49c9-b593-d223f7449a82

.travis.yml

@@ -1,5 +1,8 @@
 sudo: false
 language: php
+cache:
+  directories:
+    - node_modules
 matrix:
   include:
   - php: 5.5
@@ -73,6 +76,7 @@ before_script:
 - nvm install 4.7.2
 - npm install -g grunt-cli
 - npm install
+- npm prune
 - php --version
 - |
   # Debug PHP extensions, but not on HHVM because the command hangs indefinitely: