Update .travis.yml

Author: tmotyl

.travis.yml

@@ -1,11 +1,11 @@
 language: php
 
 php:
-  - 7.1
+  - 7.2
 
 before_script:
   - travis_retry composer self-update
   - travis_retry composer install --prefer-source --no-interaction
 
 script:
-  - phpunit
+  - phpunit