added tests for php 7.4

Author: mirko-pagliai

.travis.yml

@@ -3,6 +3,7 @@ language: php
 php:
   - 7.2
   - 7.3
+  - 7.4
 
 matrix:
   fast_finish: true
@@ -12,6 +13,8 @@ matrix:
       env: dependencies=lowest
     - php: 7.3
       env: dependencies=lowest
+    - php: 7.4
+      env: dependencies=lowest
     - php: 7.2
       env: PHPCS=1
     - php: 7.2