Support Composer 2, not 1 (#89)

Previously, this only worked with Composer 1
Composer 1 has been essentially EOL
for months, so it'd be good to support 2.
https://blog.packagist.com/deprecating-composer-1-support/

Author: kienstra

composer.json

@@ -11,7 +11,7 @@
   },
   "require-dev": {
     "10up/wp_mock": "0.4.2",
-    "dealerdirect/phpcodesniffer-composer-installer": "0.6.2",
+    "dealerdirect/phpcodesniffer-composer-installer": "0.7.1",
     "mockery/mockery": "1.3.1",
     "php-coveralls/php-coveralls": "2.2.0",
     "phpcompatibility/phpcompatibility-wp": "2.1.0",