Support Composer 2.0

Minimal changes to updated the plugin to support Composer 2.0 which is expected late May/beginning of June.

Changes are based on guidance found in:
* #108
* composer/composer#8726
* https://github.com/composer/composer/blob/master/UPGRADE-2.0.md#for-integrators-and-plugin-authors

Tested by danepowell and myself (Windows 7). Further testing would be very welcome!

Fixes 108

composer.json

@@ -24,7 +24,7 @@
   },
   "require": {
     "php": ">=5.3",
-    "composer-plugin-api": "^1.0",
+    "composer-plugin-api": "^1.0 || ^2.0",
     "squizlabs/php_codesniffer": "^2 || ^3 || 4.0.x-dev"
   },
   "require-dev": {

src/Plugin.php

@@ -124,6 +124,20 @@ public function activate(Composer $composer, IOInterface $io)
         $this->init();
     }
 
+    /**
+     * {@inheritDoc}
+     */
+    public function deactivate(Composer $composer, IOInterface $io)
+    {
+    }
+
+    /**
+     * {@inheritDoc}
+     */
+    public function uninstall(Composer $composer, IOInterface $io)
+    {
+    }
+
     /**
      * Prepares the plugin so it's main functionality can be run.
      *