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.

See #40539

Merges [40538] to the 4.5 branch.


git-svn-id: https://develop.svn.wordpress.org/branches/4.5@40549 602fd350-edb4-49c9-b593-d223f7449a82

.travis.yml

@@ -3,6 +3,8 @@ language: php
 cache:
   directories:
     - node_modules
+    - vendor
+    - $HOME/.composer/cache
 matrix:
   include:
   - php: 7.0