fix: envoi is not a real dependency

Declare envoi as a development-only dependency since it is not actually
used by any code in this library.  Its usage is depicted in the
examples so the package is retained.

Author: boite

composer.json

@@ -18,14 +18,14 @@
     ],
     "require": {
         "php": ">=5.3.0",
-        "linkorb/envoi": "^1.0",
         "linkorb/userbase-role-contracts": "^2.0",
         "psr/cache": "~1.0",
         "symfony/cache": "~3.0 || ^4.0 || ^5.0",
         "symfony/event-dispatcher-contracts": "^2.0",
         "symfony/security-core": "~2.6 || ~3.0 || ^4.0 || ^5.0"
     },
     "require-dev": {
+        "linkorb/envoi": "^1.0",
         "symfony/dotenv": "~3.0 || ^4.0 || ^5.0"
     },
     "config": {