setup changes

Two small commits here.

One modifies the Gemfile to rely on "https://rubygems.org" instead of the outdated :rubygems symbol.

Another modifies firetower#setup to disallow duplicate conf entries if someone hits the same room number multiple times.

I admin this came out of my doing so because I failed to read the full list of room options and realize that "9: done" was the way to indicate I was done selecting rooms.  I hit my primary room choice a few times thinking perhaps the setup menu selection was broken before considering it might be my ADD keeping me from seeing the "done" option.