Disable echo of command check

For some reason, `command -v brew` by itself has started to output `/usr/local/bin/brew` whenever I start a new shell (in iTerm2 2.0 on OS X 10.9.4 with zsh, oh-my-zsh etc). This fixes it, and it looks like it's the recommended way anyway:

- http://unix.stackexchange.com/a/85250/4678
- http://www.cyberciti.biz/faq/unix-linux-shell-find-out-posixcommand-exists-or-not/