Mac/Win/Linux 升级和回退composer版本以及解决Could not parse version constraint ~4.*或^1.0.2

laravel执行composer update出现如下错误：

  [RuntimeException]                                                                                                          
  Could not load package mews/purifier in https://packagist.phpcomposer.com: [UnexpectedValueException] Could not parse vers  
  ion constraint ~4.*: Invalid version string "~4.*"                                                                          

  [UnexpectedValueException]                                              
  Could not parse version constraint ~4.*: Invalid version string "~4.*" 

原因是当前composr版本过低，解决办法如下：

# 升级composer

sudo composer self-update

提示如下：

Updating to version 1.10.13 (stable channel).
   Downloading (100%)         
Use composer self-update --rollback to return to version 1.8.5

这里您可以看到可以使用如下命令回退版本：

composer self-update --rollback