外键需要未签名: http://laravel.com/docs/schema#foreign-keys

将远程表的密钥更改为未签名

Change the key of the remote table to unsigned

public function up() {
Schema::create('clases_inventario', function($t) {
            $t->increments('id');
            $t->integer('grupos_inventario_id')->unsigned();
            $t->integer('laboratorio_id');
            $t->string('codigo', 4);
            $t->string('descripcion', 64);
            $t->boolean('estado')->default(true);
            $t->timestamps();
        });
}