Update client compatibility for newer versions PHP >= 7.2, <= 8.1

[nacholibre]

I've update a project from symfony 4.4 to symfony 5.4 and I've started to get errors like these:

Compile Error: Declaration of Smalot\\Cups\\Transport\\Client::sendRequest(Psr\\Http\\Message\\RequestInterface $request) must be compatible with Psr\\Http\\Client\\ClientInterface::sendRequest(Psr\\Http\\Message\\RequestInterface $request)

Changelog for this PR:

1. Drop support for php 5.6, 7 and 7.1 - this was needed because newer versions of the packages required new php versions. I figured it's better to support php 8, so I've decided to drop support of these older php versions. Php 7.1 is EOF anyway.
2. Update package used for testing atoum/atoum
3. Add travis tests for all of the php versions - 7.2, 7.3, 7.4, 8.0 and 8.1
4. Add version requirements for packages, because * can cause problems, like the ones we experience with the Compile Error:

[nacholibre]

@smalot This is ready for review, not sure why the travis CI build doesn't appear here, but in my fork - it's passing - https://github.com/nacholibre/cups-ipp/runs/9135650354

Thanks

[ro0NL]

ping @smalot

[ro0NL]

see https://packagist.org/packages/etrias/cups-ipp