Prefer lowest dependencies (#20)

Author: Ekman

.travis.yml

@@ -7,15 +7,13 @@ php:
   - 7.3
   - 7.4snapshot
   - nightly
-  - hhvm
 
 matrix:
   allow_failures:
   - php: 7.4snapshot
   - php: nightly
-  - php: hhvm
 
-install: composer install --no-interaction --classmap-authoritative --no-suggest --ignore-platform-reqs
+install: composer update --no-interaction --classmap-authoritative --no-suggest --ignore-platform-reqs --prefer-lowest
 
 script: vendor/bin/phpunit --configuration phpunit.xml