Update .travis.yml

Author: eldadfux

.travis.yml

@@ -4,6 +4,7 @@ php:
 - 7.1
 - 7.2
 - 7.3
+- 7.4
 
 notifications:
   email:
@@ -12,4 +13,4 @@ notifications:
 before_script: composer install
 
 script:
-- vendor/bin/phpunit --configuration phpunit.xml
+- vendor/bin/phpunit --configuration phpunit.xml