Composer: tweak PHPCS version constraint

Some of the early PHPCS 3.x versions did not support external standards correctly, so using this plugin with those would always run into trouble, so let's just make it explicit that `3.0.0`, `3.0.1` and `3.0.2` are not supported.

composer.json

@@ -25,7 +25,7 @@
   "require": {
     "php": ">=5.3",
     "composer-plugin-api": "^1.0 || ^2.0",
-    "squizlabs/php_codesniffer": "^2.0 || ^3.0 || ^4.0"
+    "squizlabs/php_codesniffer": "^2.0 || ^3.1.0 || ^4.0"
   },
   "require-dev": {
     "composer/composer": "*",