Updated library versions & updated README to reflect minimum versions

Author: jdomenechb

README.md

@@ -1,6 +1,6 @@
 # ReactPHP Symfony Server
 
-This library wraps a ReactPHP server over a Symfony project installation.
+This library wraps a ReactPHP server over a Symfony 4 project installation.
 
 ## Installation (using composer)
 
@@ -28,6 +28,10 @@ The library takes the `public/index.php` file from the project root, and reads i
 
 Once executed, requests are made directly to the server and translated to Symfony requests, which are dealt then by the Symfony Kernel. The Kernel provides afterwards a response, which is translated back to a response the ReactPHP server can understand.
 
+**Q. Which versions of PHP and Symfony are supported?**
+
+PHP >= 7.1 and Symfony 4.*.
+
 **Q. Does it serve static assets too?**
 
 Yes, the library has basic support for serving static assets, like images, CSS ans JS under the `public` folder of your project.

composer.json

@@ -11,15 +11,15 @@
     ],
     "minimum-stability": "stable",
     "require": {
-        "php": ">=7.0",
+        "php": ">=7.1",
         "react/http": "^0.8.0",
-        "symfony/http-foundation": "*",
-        "symfony/console": "*",
-        "symfony/http-kernel": "*",
+        "symfony/http-foundation": "^4.0",
+        "symfony/console": "^4.0",
+        "symfony/http-kernel": "^4.0",
         "ralouphie/mimey": "^2.1"
     },
     "require-dev": {
-        "friendsofphp/php-cs-fixer": "^2.13"
+        "friendsofphp/php-cs-fixer": "^2.0"
     },
     "autoload": {
       "psr-4": {