Update .travis.yml

Author: imrealashu

.travis.yml

@@ -4,11 +4,4 @@ php:
   - 7.0
   - 7.1
 
-env:
-  matrix:
-    - COMPOSER_FLAGS="--prefer-lowest"
-    - COMPOSER_FLAGS=""
-
-before_script:
-  - travis_retry composer self-update
-  - travis_retry composer update ${COMPOSER_FLAGS} --no-interaction --prefer-source
+script: vendor/bin/phpunit