Add support for Composer 2

[villfa]

Before this PR we couldn't install scriptsdev with Composer 2:

$ ./composer.phar --version
Composer version 2.0.0-alpha2 2020-06-24 21:36:18
$ ./composer.phar require neronmoon/scriptsdev --dev
                                                                                                                                          
  [InvalidArgumentException]                                                                                                              
  Package neronmoon/scriptsdev at version  has a PHP requirement incompatible with your PHP version, PHP extensions and Composer version  
                                                                                                                                          

Closes #18

[villfa]

Hi @neronmoon,
Since Composer 2 has been released it would be great if you could review this PR.

Thanks in advance

[rickgoemans]

I'm also waiting for this merge!

[neronmoon]

Thank you for PR!