Update .travis.yml

.travis.yml

@@ -3,6 +3,7 @@ services:
 before_script:
   - echo "USE mysql;\nUPDATE user SET password=PASSWORD('dev') WHERE user='root';\nFLUSH PRIVILEGES;\n" | mysql -u root
 install:
+  - composer require --dev phpunit/phpunit ^5.7
   - phpunit --version
 language: php
 php: