Revert "update composer dependencies"

This reverts commit 50225db.

Author: tschortsch

composer.json

@@ -24,10 +24,10 @@
 		"email": "contact@liip.ch"
 	},
 	"require-dev": {
-		"squizlabs/php_codesniffer": "^3.7",
+		"squizlabs/php_codesniffer": "^3.6",
 		"wp-coding-standards/wpcs": "dev-develop",
-		"dealerdirect/phpcodesniffer-composer-installer": "^0.7.2",
-		"wp-phpunit/wp-phpunit": "^6.0",
+		"dealerdirect/phpcodesniffer-composer-installer": "^0.7.1",
+		"wp-phpunit/wp-phpunit": "^5.8",
 		"yoast/phpunit-polyfills": "^1.0"
 	},
 	"minimum-stability": "dev",