Update corral.json in preparation for 0.4.0 release

Author: ponylang-main

corral.json

@@ -7,7 +7,7 @@
     "homepage": "https://github.com/ponylang/http_server",
     "license": "BSD-2-Clause",
     "documentation_url": "https://ponylang.github.io/http_server/",
-    "version": "0.3.3",
+    "version": "0.4.0",
     "name": "http_server"
   },
   "deps": [