Fix Travis CI: composer install --dev option is deprecated

Author: Ayesh

.travis.yml

@@ -7,7 +7,7 @@ php:
 
 before_script:
   - composer self-update
-  - travis_retry composer install --prefer-source --no-interaction --dev
+  - travis_retry composer install --prefer-source --no-interaction
 script:
   - vendor/bin/phpunit --coverage-clover=coverage.xml
 after_success: