Update .travis.yml

Author: Nico Oelgart

.travis.yml

@@ -4,7 +4,9 @@ php:
   - 7.0
   - 7.1
 
-script: phpunit --coverage-clover=coverage.clover
+script:
+  - composer install --dev
+  - phpunit --coverage-clover=coverage.clover
 
 after_script:
   - wget https://scrutinizer-ci.com/ocular.phar