Test against PHP 8.0

* Drop < PHP 7.3 in TravisCI
* Bump eloquent/phony-phpunit to 7.1
* Bump phpunit to 9.5

Author: jbboehr

.travis.yml

@@ -1,17 +1,17 @@
 language: php
 
 php:
-  - 7.0
-  - 7.1
-  - 7.2
+  - 7.3
+  - 7.4
+  - 8.0
   - nightly
 
 matrix:
   fast_finish: true
   allow_failures:
     - php: nightly
   include:
-    - php: 7.0
+    - php: 7.3
       env: 'COMPOSER_FLAGS="--prefer-stable --prefer-lowest"'
 
 before_script:

composer.json

@@ -14,8 +14,8 @@
         "psr/http-server-middleware": "^1.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^6.0",
-        "eloquent/phony-phpunit": "^3.0",
+        "phpunit/phpunit": "^9.5",
+        "eloquent/phony-phpunit": "^7.1.0",
         "eloquent/liberator": "^2.0"
     },
     "autoload": {