Updates .travis.yml.

Author: mvdnbrk

.travis.yml

@@ -1,16 +1,17 @@
 language: php
 
-dist: trusty
-
 php:
   - 7.2
   - 7.3
 
-sudo: false
+env:
+  matrix:
+    - COMPOSER_FLAGS="--prefer-lowest"
+    - COMPOSER_FLAGS=""
 
 before_script:
   - travis_retry composer self-update
-  - travis_retry composer update --no-interaction --prefer-dist
+  - travis_retry composer update ${COMPOSER_FLAGS} --no-interaction --prefer-source
 
 script:
   - vendor/bin/phpunit --coverage-text --coverage-clover=coverage.clover