Composer: update PHPCS Composer plugin dependency

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

@@ -29,7 +29,7 @@
     "php": ">=5.2"
   },
   "require-dev": {
-    "dealerdirect/phpcodesniffer-composer-installer": "^0.5.0",
+    "dealerdirect/phpcodesniffer-composer-installer": "^0.6",
     "phpcompatibility/phpcompatibility-wp": "^2.0",
     "wp-coding-standards/wpcs": "^2.0"
   },