Update slim/psr7 dependencies

Author: odan

composer.json

@@ -29,7 +29,7 @@
     },
     "require-dev": {
         "phpunit/phpunit": "^7.0 || ^8.0",
-        "slim/psr7": "^1",
+        "slim/psr7": "^0.6 || ^1",
         "squizlabs/php_codesniffer": "^3.5"
     },
     "scripts": {