dependency versions

[Firtzberg]

#8, which is fine, got me thinking.
I'm using this package in HuddleService, which uses Guzzle 6. I upgraded this package and ended up with version 2.2.1, although version 2.2.2 is available. Having a Guzzle version 7 constraint is too hard. I think we're compatible with versions from 4 to 7.
"guzzlehttp/guzzle": "^4|^5|^6|^7"
Supporting all these versions is tricky, to make sure we don't break anything we should use automated tools to check the compatibility on each PR. I've used Travis in the past. Here is an example where I use it to check a package compatibility with Laravel versions from 5.5 to 8 with different PHP versions.

[nico-spada-virtasant]

totally agree. good idea, maybe we can split this in 2 PR, one to add multiple guzzle versions and another to add Travis.