Use dependency injection in migrations

src/Install/Migration.php

@@ -0,0 +1,16 @@
+<?php namespace Flarum\Install;
+
+use Illuminate\Database\Schema\Builder;
+
+abstract class Migration
+{
+    /**
+     * @var Builder
+     */
+    protected $schema;
+
+    public function __construct(Builder $builder)
+    {
+        $this->schema = $builder;
+    }
+}