Detect dev/beta versions and provide appropriate instruction

Author: xelaris

src/Symfony/Installer/NewCommand.php

@@ -182,6 +182,15 @@ protected function checkSymfonyVersionIsInstallable()
             ));
         }
 
+        if (preg_match('/^(\d\.\d).*-(dev|beta)\d*$/', $this->version, $matches)) {
+            throw new \RuntimeException(sprintf(
+                "You are trying to install an unstable version (%s).\n".
+                "Please use composer to install such versions:\n\n".
+                "composer create-project symfony/framework-standard-edition %s \"%s.*\" --stability=%s",
+                $this->version, $this->projectDir, $matches[1], $matches[2]
+            ));
+        }
+
         return $this;
     }