Rename bash completion script

This completion script must wrap homebrew core's completion function
(_brew) in order to work (since it's adding completions for subcommands
of brew). As such, homebrew core's completion script must be loaded
first. The bash-completion package (at least, v2) seems to load them
in globbed file order. The homebrew core completion script is named
`brew`. Thus, in order for this completion script to be loaded after
core, it must sort _after_ `brew` in a listing. Dropping the leading
underscore pushes this script after brew core. Also, swapped the
mid-word underscore for a hyphen to match the style that brew-cask
uses/used.