fix tests on travis

Author: splitbrain

.travis.yml

@@ -1,12 +1,14 @@
 language: php
 sudo: false
+dist: trusty
 php:
  - 5.4
  - 5.5
  - 5.6
  - 7.0
  - 7.1
  - 7.2
+ - 7.3
 before_script:
  - composer self-update
  - composer install --prefer-source --no-interaction --dev