Update psr/http-message requirement from ^1.0 to ^1.0 || ^2.0 (#196)

Updates the requirements on [psr/http-message](https://github.com/php-fig/http-message) to permit the latest version.
- [Release notes](https://github.com/php-fig/http-message/releases)
- [Changelog](https://github.com/php-fig/http-message/blob/master/CHANGELOG.md)
- [Commits](php-fig/http-message@1.0...2.0)

---
updated-dependencies:
- dependency-name: psr/http-message
  dependency-type: direct:development
...

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composer.json

@@ -74,7 +74,7 @@
   "require-dev": {
     "psr/http-client": "^1.0",
     "psr/http-factory": "^1.0",
-    "psr/http-message": "^1.0",
+    "psr/http-message": "^1.0 || ^2.0",
     "symfony/dotenv": "^3.4|^4.0|^5.0",
     "symfony/http-client-contracts": "^1.1|^2.0",
     "symfony/http-client": "^4.4|^5.0",