update omnipay readme to reflect support for 3x

[welrachid]

Hi
just wanted to let you know that currently the omnipay community list shows that you only support 2x. from reading your readme it says you support 3.x

maybe you could make a PR to them to update their readmefile?

i could ofcourse do it, but since im not the author i can't take the credit/blame for that ;)

[thomasbnielsen]

Thank you for the info and i have made a PR to their readme file