made gff2bed work as stand-alone if --do-not-sort specified

Your gff2bed script is very useful, but as a user I just want to be able to download and use this script without the rest of your whole package, and I don't care about sorting. I moved the lines that check to see whether sort-bed is installed so that you don't get an error about not having sort-bed installed when the script isn't even going to call it. Hope this helps!