Build/Test Tools: Add Composer files to the cache on Travis.

The Travis cache is specific to the branch and language version (PHP version), so this should speed up each subsequent build once the cache is primed.

Props netweb

Fixes #40539


git-svn-id: https://develop.svn.wordpress.org/trunk@40538 602fd350-edb4-49c9-b593-d223f7449a82

.travis.yml

@@ -4,6 +4,8 @@ cache:
   apt: true
   directories:
     - node_modules
+    - vendor
+    - $HOME/.composer/cache
 env:
   global:
     - WP_TRAVISCI=travis:phpunit