change install command

the master branch was upgrade to 5.6, so

Author: ac1982

installation.md

@@ -48,7 +48,7 @@ Laravel 利用 [Composer](https://getcomposer.org) 来管理依赖。所以，
 
 或者，你还可以通过在终端中运行 `create-project` 命令来安装 Laravel：
 
-    composer create-project --prefer-dist laravel/laravel blog
+    composer create-project --prefer-dist laravel/laravel blog "5.5.*"
 
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