Add support for Composer version 2

Cherry picking from #309

Author: Ayesh

composer.json

@@ -15,10 +15,10 @@
   ],
   "require": {
     "php": ">=5.3.0",
-    "composer-plugin-api": "^1.0"
+    "composer-plugin-api": "^1.0 || ^2.0"
   },
   "require-dev": {
-    "composer/composer": "~1.0",
+    "composer/composer": "~1.0 || ~2.0",
     "phpunit/phpunit": "~4.6"
   },
   "autoload": {