Loosen the validation for the URL field of VCS repositories

The URL of the VCS repository may not match the uri pattern of the JSON
schema spec, for instance when using a SSH URL of the repository.

res/composer-schema.json

@@ -596,7 +596,7 @@
             "required": ["type", "url"],
             "properties": {
                 "type": { "type": "string", "enum": ["vcs", "github", "git", "gitlab", "git-bitbucket", "hg", "hg-bitbucket", "fossil", "perforce", "svn"] },
-                "url": { "type": "string", "format": "uri" },
+                "url": { "type": "string" },
                 "no-api": { "type": "boolean" },
                 "secure-http": { "type": "boolean" },
                 "svn-cache-credentials": { "type": "boolean" },