Made bash script more portable

Hey,

While I was setting up duiadns on my server, I noticed that the interpreter for the script was set to /bin/bash which does not exist on my machine. Fortunately, the script is also interpretable by sh(1) which is available out of the box on most (if not all?) unix machines.

Also the script now checks for wget and curl and uses whichever is available. This allows me to move this script to a different machine without a worry.

Cheers,

P.S. I don't want to come over all pedantic.