Allow installing on PHP 8. (#45)

* Allow installing on PHP 8.

* Update .travis.yml

Co-authored-by: Tobias Nyholm <tobias.nyholm@gmail.com>

Author: ADmad

.travis.yml

@@ -9,6 +9,8 @@ cache:
 php:
     - 7.2
     - 7.3
+    - 7.4
+    - 'nightly'
 
 env:
     global:
@@ -29,7 +31,12 @@ before_install:
     - iphpenv config-rm xdebug.ini || true
 
 install:
-    - travis_retry composer update --prefer-dist --no-interaction
+  - |
+      if [[ $TRAVIS_PHP_VERSION == 'nightly' ]]; then
+        travis_retry composer update --prefer-dist --no-interaction --ignore-platform-reqs
+      else
+        travis_retry composer update --prefer-dist --no-interaction
+      fi
 
 script:
     - $TEST_COMMAND --testsuite $SUITE

composer.json

@@ -14,8 +14,8 @@
         }
     ],
     "require": {
-        "php": "^7.2",
-        "phpunit/phpunit": "^8.0",
+        "php": "^7.2 || ^8.0",
+        "phpunit/phpunit": "^8.0 || ^9.3",
         "psr/http-message": "^1.0"
     },
     "require-dev": {