PHP 8 support

- added php 8 to travis config
- allow phpunit 9 to be used

Author: adlenton

.travis.yml

@@ -10,6 +10,7 @@ php:
   - 7.2
   - 7.3
   - 7.4
+  - nightly #php 8
 
 matrix:
   include:

composer.json

@@ -28,6 +28,6 @@
         }
     },
     "require-dev": {
-        "phpunit/phpunit": ">=4.8 <=9"
+        "phpunit/phpunit": ">=4.8 <=10"
     }
 }