Don't test on lowest depends

.travis.yml

@@ -12,18 +12,13 @@ cache:
   directories:
   - "$HOME/.composer/cache/files"
 
-env:
-  matrix:
-    - COMPOSER_FLAGS="--prefer-lowest"
-    - COMPOSER_FLAGS=""
-
 matrix:
   allow_failures:
     - php: nightly
 
 before_script:
     - travis_retry composer self-update
-    - travis_retry composer update ${COMPOSER_FLAGS} --no-interaction --no-suggest --prefer-source
+    - travis_retry composer update --no-interaction --no-suggest --prefer-source
 
 script:
     - vendor/bin/phpunit --testdox