Composer: prevent a lock file from being created

Composer 1.10.0 introduced a `lock` config option, which, when set to `false` will prevent a `composer.lock` file from being created and will ignore it when one exists.

As for this package, none of the dependencies need to be locked, we can use this option.

It also makes life easier for contributors as they don't have to remember that for this repo they should use `composer update` instead of `composer install`. Both will now work the same.

Refs:
https://getcomposer.org/doc/06-config.md#lock

composer.json

@@ -50,7 +50,8 @@
 			"dealerdirect/phpcodesniffer-composer-installer": true,
 			"composer/installers": true
 		},
-		"classmap-authoritative": true
+		"classmap-authoritative": true,
+		"lock": false
 	},
 	"scripts": {
 		"lint": [