README update bash
README update bash

Bash 4.x is required but this wasn't so clear in README

This may address https://github.com/Homebrew/homebrew-command-not-found/issues/27 and https://github.com/Homebrew/homebrew-command-not-found/issues/26

bfontaine/homebrew-command-not-found,Homebrew/homebrew-command-not-found,Homebrew/homebrew-command-not-found,bfontaine/homebrew-command-not-found