Prevent fatal error when composer hasn't been run

composer.json

@@ -31,5 +31,8 @@
             "Grav\\": "system/src/"
         },
         "files": ["system/defines.php"]
+    },
+    "archive": {
+        "exclude": ["VERSION"]
     }
 }

index.php

@@ -4,12 +4,16 @@ namespace Grav;
 if (version_compare($ver = PHP_VERSION, $req = '5.4.0', '<')) {
     exit(sprintf('You are running PHP %s, but Grav needs at least <strong>PHP %s</strong> to run.', $ver, $req));
 }
+$autoload = __DIR__ . '/vendor/autoload.php';
+if (!is_file($autoload)) {
+    exit('Please run: <i>composer install -o</i>');
+}
 
 use Grav\Common\Grav;
 use Grav\Common\Debugger;
 
 // Register the auto-loader.
-$loader = require_once __DIR__ . '/vendor/autoload.php';
+$loader = require_once $autoload;
 
 if (!ini_get('date.timezone')) {
     date_default_timezone_set('UTC');