Small README update for sbt

[jsburke]

Hi Kevin,

Thanks for your work. I'm getting my feet wet in rfuzz so thank you for your hard work. I was following the readme, and got stuck installing sbt despite having worked with Chisel based cores before (new Ubuntu install for my computer). While the link you point to for installing sbt lists the correct steps for installation, the README is slightly out of date for it now because bintray has been sunsetted. It's pretty minor and it only took a few minutes for me to work around, but you may want to update the sbt install portion of the README to bring it up to date.

[ekiwi]

Thanks for letting me know. I will look into it.