README: explicitly state required cucumber version

Hi,

I just spent some time trying to use this nice little plugin, until I realized cucumber version 1.1.5 is not just an example, it's a requirement (earlier versions either fail with an InvocationException or simply can't locate the cucumber Backend), and my project used v1.0.8 :poop: .

So I just added this explicit statement to the README, to spare future frustration. 
It may be worth throwing an indicative exception in case a wrong version is used - didn't go that far, sorry..

Thanks!