Composer: update PHPCS Composer plugin dependency (#121)

The DealerDirect Composer plugin has just released version `0.6.0`.
As Composer treats minors < 1.0 as majors, updating to this version requires an update to the `composer.json` requirements.

> For pre-1.0 versions it also acts with safety in mind and treats `^0.3` as `>=0.3.0 <0.4.0`.

Refs:
* https://github.com/Dealerdirect/phpcodesniffer-composer-installer/releases/tag/v0.6.0
* https://getcomposer.org/doc/articles/versions.md#caret-version-range-

composer.json

@@ -38,7 +38,7 @@
         "squizlabs/php_codesniffer": "^3.1"
     },
     "require-dev": {
-        "dealerdirect/phpcodesniffer-composer-installer": "^0.4.4 || ^0.5",
+        "dealerdirect/phpcodesniffer-composer-installer": "^0.4.4 || ^0.5 || ^0.6",
         "phpunit/phpunit": "^5.0 || ^6.5",
         "sirbrillig/phpcs-import-detection": "^1.1",
         "limedeck/phpunit-detailed-printer": "^3.1",