Added php-test-helpers to the vendors directory

Seemed easier than trying to figure out how to force the prompt to accept the
unknown IP (perhaps not possible at all as it is part of SSH?). `yes` didn't
work, unfortunately.

.travis.yml

@@ -10,8 +10,7 @@ matrix:
 
 install:
   - composer install
-  - yes | git clone git@github.com:php-test-helpers/php-test-helpers.git
-  - cd php-test-helpers
+  - cd vendors/php-test-helpers
   - phpize
   - ./configure --enable-test-helpers
   - make