update Travis

.travis.yml

@@ -1,3 +1,6 @@
+# use ubuntu trusty for old PHP versions
+dist: trusty
+
 language: php
 
 php:
@@ -6,6 +9,7 @@ php:
   - 7.0
   - 7.1
   - 7.2
+  - 7.3
 
 # faster builds on new travis setup not using sudo
 sudo: false
@@ -17,7 +21,7 @@ cache:
 
 install:
   - travis_retry composer install --prefer-dist --no-interaction
-  - travis_retry composer require "phpunit/phpunit:^4.8.27|^5.0|^6.0" --update-with-dependencies
+  - travis_retry composer require "phpunit/phpunit:4.8.27|^5.0|^6.0" --update-with-dependencies
 
 script:
   - vendor/bin/phpunit $PHPUNIT_FLAGS