fix(composer): add autoload for real

Well, that’s embarrassing.

Author: Florens Verschelde

composer.json

@@ -11,5 +11,10 @@
   ],
   "require": {
     "php": ">=5.4.0"
+  },
+  "autoload": {
+    "psr-4": {
+      "Kirby\\StaticBuilder\\": "src/"
+    }
   }
 }