Laravel-admin: 无法更新到1.5.18版本以上

[sayhei]

• Laravel Version: 5.5.*
• PHP Version:
• Laravel-admin: 1.5.15

Description:

在执行更新操作时，报了如下错误
Problem 1
- The requested package encore/laravel-admin ^v1.6 exists as encore/laravel-admin[1.1.x-dev, 1.2.x-dev, 1.3.x-dev, 1.4.x-dev, 1.5.x-dev, dev-analysis-8AvKDk, dev-analysis-q14nMj, dev-develop, dev-master, v1.1.6, v1.2.10, v1.2.6, v1.2.8, v1.2.9, v1.3.3, v1.3.4, v1.4.1, v1.4.4, v1.4.5, v1.4.6, v1.5.10, v1.5.11, v1.5.12, v1.5.13, v1.5.14, v1.5.15, v1.5.5, v1.5.6, v1.5.7, v1.5.8, v1.5.9] but these are rejected by your constraint.

Steps To Reproduce:

[jxlwqq]

教程：https://laravel-china.org/wikis/16722