prefer-stable isn't a valid option for install

.travis.yml

@@ -10,13 +10,10 @@ php:
 matrix:
   allow_failures:
     - php: 7.0
-  include:
-    - php: 5.4
-      env: 'COMPOSER_FLAGS="--prefer-stable --prefer-lowest"'
 
 sudo: false
 
-install: travis_retry composer install ${COMPOSER_FLAGS} --no-interaction --prefer-source
+install: travis_retry composer install --no-interaction --prefer-source
 
 before_script: git reset --hard HEAD