Update travis configuration

Author: Michael Lundbøl

.travis.yml

@@ -1,8 +1,10 @@
 language: php
 php:
-- '5.5'
+- '7.1'
+- '7.2'
+- '7.3'
 before_script:
 - travis_retry composer self-update
-- travis_retry composer install --prefer-source --no-interaction
+- travis_retry composer install --prefer-lowest --no-interaction
 notifications:
   email: false