Run Travis CI builds with different versions of dependencies

Author: chadicus

.travis.yml

@@ -1,13 +1,18 @@
+sudo: false
 language: php
 php:
   - 7.0
   - 7.1
   - 7.2
   - nightly
-install: composer install
-script: ./vendor/bin/phpunit
-after_success: ./vendor/bin/php-coveralls -v
+env:
+  - PREFER_LOWEST="--prefer-lowest --prefer-stable"
+  - PREFER_LOWEST=""
 matrix:
   fast_finish: true
   allow_failures:
      - php: nightly
+before_script:
+  - composer update $PREFER_LOWEST
+script: ./vendor/bin/phpunit
+after_success: ./vendor/bin/php-coveralls -v