Patch to backported feature from v2.2

NOTE: set api_key_id to nullable to fix issues regarding `SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails` errors as described in #48

src/migrations/2014_06_12_084423_create_api_keys_table.php

@@ -28,7 +28,7 @@ public function up()
 
         Schema::create('api_logs', function (Blueprint $table) {
             $table->increments('id');
-            $table->integer('api_key_id')->unsigned();
+            $table->integer('api_key_id', false, true)->nullable();
             $table->string('route', 50);
             $table->string('method', 6);
             $table->text('params');