Explicitly bundle Carbon library

We have used this transitive dependency (via illuminate/support)
for a while, so let's make this explicit.

Incidentally, we now also explicitly require version 2.x - the
previous 1.x branch will no longer be supported after the
upcoming upgrade to Laravel 6.

Refs #2055.

composer.json

@@ -66,6 +66,7 @@
         "middlewares/base-path-router": "^0.2.1",
         "middlewares/request-handler": "^1.2",
         "monolog/monolog": "^1.16.0",
+        "nesbot/carbon": "^2.0",
         "nikic/fast-route": "^0.6",
         "psr/http-message": "^1.0",
         "psr/http-server-handler": "^1.0",