Upgrade requirements for psr/http-message

[tadasrimkus]

Please update requirements for psr/http-message to support versions above ^2.0.

Change to composer.json

 "psr/http-message": "~1.0 || ^2.0",

I doubt this small change will have an impact on the library itself, but it allows us to use non forked version, and avoids conflicts with other packages.

[jcchavezs]

I suggest you to open a PR trying that out. This library has enough tests (unit, integration, e2e) to detect breaking changes.