Revert "Re add prefer-lowest build"

This reverts commit a97e055ed3.

.travis.yml

@@ -4,15 +4,9 @@ php:
   - 7.1
   - 7.2
 
-env:
-  matrix:
-    - PREFER_LOWEST="--prefer-lowest"
-    - PREFER_LOWEST=""
-
 before_script:
   - composer self-update
-  - composer update --no-suggest $PREFER_LOWEST
-  - composer install --no-interaction
+  - composer install --prefer-source --no-interaction
 
 script: 
 - vendor/bin/phpunit --coverage-clover=coverage.xml