Allow for the fact $this->plugins_locked is an object

Before: we were trying to loop through `$this->plugins_locked` as if it
was an array (as previously we'd been using `foreach` with key/value
pairs). However, it's actually an object, and foreach-ing an object
produces a key-value array of the visible properties of that object.

Now: we use get_object_vars to get the visible properties as an array,
then use array_keys on that.

src/Modules/Plugin.php

@@ -239,7 +239,7 @@ public function install($internal = false)
 		$gitignore = new \Dxw\Whippet\Git\Gitignore($this->project_dir);
 		$ignores = $gitignore->get_ignores();
 
-		foreach (array_keys($this->plugins_locked) as $dir) {
+		foreach (array_keys(get_object_vars($this->plugins_locked)) as $dir) {
 			$plugin_dir = "/wp-content/plugins/" . strval($dir) . "\n";
 
 			if (array_search($plugin_dir, $ignores) === false) {