Prefer source so we get the tests.

.travis.yml

@@ -34,7 +34,7 @@ before_script:
 # Install composer dependencies
   - composer validate
   - composer require --no-update silverstripe/recipe-cms:4.3.x-dev
-  - composer install --prefer-dist --no-interaction --no-progress --no-suggest --optimize-autoloader --verbose --profile
+  - composer install --prefer-source --no-interaction --no-progress --no-suggest --optimize-autoloader --verbose --profile
   - if [[ $PHPCS_TEST ]]; then composer global require squizlabs/php_codesniffer:^3 --prefer-dist --no-interaction --no-progress --no-suggest -o; fi
 
 script: