Support PHPUnit 9 (#50)

Author: Nyholm

.travis.yml

@@ -7,13 +7,14 @@ cache:
         - $HOME/.composer/cache/files
 
 php:
-    - 7.3
+    - 7.4
 
 env:
     - SUITE="curl" PACKAGE="php-http/curl-client:dev-master zendframework/zend-diactoros"
     - SUITE="Socket" PACKAGE="php-http/socket-client:dev-master php-http/client-common"
     - SUITE="Guzzle6" PACKAGE="php-http/guzzle6-adapter:dev-master"
     - SUITE="Guzzle" PACKAGE="guzzlehttp/guzzle:dev-master"
+    - SUITE="Guzzle" PACKAGE="guzzlehttp/guzzle:dev-master phpunit/phpunit:^8.5.8"
     - SUITE="React" PACKAGE="php-http/react-adapter:dev-master"
     - SUITE="Buzz" PACKAGE="php-http/buzz-adapter:dev-master"
     - SUITE="CakePHP" PACKAGE="php-http/cakephp-adapter:dev-master"

composer.json

@@ -15,10 +15,10 @@
         }
     ],
     "require": {
-        "php": "^7.1",
-        "phpunit/phpunit": "^7.0 || ^8.0",
-        "php-http/message": "^1.0",
-        "guzzlehttp/psr7": "^1.0",
+        "php": "^7.1 || ^8.0",
+        "phpunit/phpunit": "^7.5.20 || ^8.5.8 || ^9.3.11",
+        "php-http/message": "^1.0 || ^2.0",
+        "guzzlehttp/psr7": "^1.7",
         "th3n3rd/cartesian-product": "^0.3"
     },
     "suggest": {