Add support for PHP 8

Author: JaZo

Diff for: .travis.yml

@@ -1,11 +1,10 @@
-dist: trusty
 language: php
 
 php:
-  - 7.1
   - 7.2
   - 7.3
   - 7.4
+  - 8.0
 
 ## Cache composer
 cache:
@@ -14,7 +13,7 @@ cache:
 
 matrix:
   include:
-    - php: 7.1
+    - php: 7.2
       env: 'COMPOSER_FLAGS="--prefer-stable --prefer-lowest"'
 
 before_script:

Diff for: composer.json

@@ -22,7 +22,7 @@
         }
     ],
     "require": {
-        "php": "^7.1",
+        "php": "^7.2|^8.0",
         "danielstjules/stringy": "^3.1",
         "php-http/discovery": "^1.4",
         "php-http/mock-client": "^1.2",
@@ -31,7 +31,7 @@
     "require-dev": {
         "friendsofphp/php-cs-fixer": "^2.17",
         "guzzlehttp/guzzle": "^7.3",
-        "phpunit/phpunit": "^7.0|^8.0"
+        "phpunit/phpunit": "^8.0|^9.0"
     },
     "provide": {
         "php-http/async-client-implementation": "1.0",