refactor: add php versions in travis job

Author: kouinkouin

.travis.yml

@@ -6,6 +6,11 @@ php:
   - '5.5'
   - '5.6'
   - '7.0'
+  - '7.1'
+  - '7.2'
+  - '7.3'
+  - '7.4'
+  - '8.0'
 
 before_install:
   - composer self-update