Shell-escape an argument to system (#146)
Shell-escape an argument to system (#146)

This commit fixes #145, where the version number argument was not being escaped before being passed to `system`, which resulted in a syntax error from `sh` if the version number contained punctuation or other shell characters.
buo/brew-cask-upgrade,buo/homebrew-cask-upgrade,buo/homebrew-cask-upgrade,buo/brew-cask-upgrade