Update .travis.yml

.travis.yml

@@ -26,7 +26,6 @@ matrix:
       env: CODECOVERAGE=1 DEFAULT=0
 
 before_script:
-  - composer self-update
   - composer install --prefer-dist --no-interaction
 
   - if [[ $PHPCS != 1 ]]; then composer require phpunit/phpunit:"^5.7|^6.0"; fi