Make bash completion script work on Macs

I started using a Mac for development recently, and boom's bash completion script didn't work there at all.

The problem was that `sed` on a Mac only accepts "basic" regexes; the completion script was using the one-or-more operator `+`, which is only available in "extended" regexes. I replaced the `+` with the uglier but more compatible `{1,}` and the script now works.