迁移laravel 5.1中的外键错误

我已经迁移了这个代码

 public function up()
{
    Schema::create('TechnicianGroup', function(Blueprint $table){
        $table->increments('id')->unsigned();
        $table->string('name', 255);
        $table->string('description', 64);
        $table->string('token', 255);
        $table->timestamps();
    });
    Schema::create('Site', function(Blueprint $table){
        $table->increments('id');
        $table->string('name', 255);
        $table->string('token', 255);
        $table->timestamps();
    });
    Schema::create('technician', function (Blueprint $table) {
        $table->increments('id');
        $table->string('name', 50);
        $table->string('username', 50);
        $table->string('password', 255);
        $table->string('email', 255);
        $table->string('mobile', 11);
        $table->string('avatar_url', 256);
        $table->string('job_title', 100);
        $table->integer('site_id');
        $table->foreign('site_id')->references('id')->on('site')->onDelete('cascade');
        $table->integer('group_id');
        $table->boolean('is_active');
        $table->string('token', 255);
        $table->timestamps();
    });
}

当我执行migarate 时

php artisan migrate

结果是

[照明''数据库''查询异常]SQLSTATE[HY000]：一般错误：1005无法创建表"support"#sql-a3c_140'（错误号：150）（sql：更改表technician添加约束technician_site_id_external外键（site_id）引用ces site（id））[PDO例外]SQLSTATE[HY000]：一般错误：1005无法创建表"support"#sql-a3c_140'（错误号：150）