bug: HOMEBREW_DEBUG prints to stdout [patch]

I usually have HOMEBREW_DEBUG set in my environment (try to intercept formula errors rather than fail, etc.).
I just realized:
```sh
~> SOMEWHERE="$(brew --prefix pcre 2>/dev/null)"
~> echo $SOMEWHERE
/usr/local/Library/brew.rb (Formulary::StandardLoader): loading /usr/local/Library/Formula/pcre.rb /usr/local/opt/pcre
```
...would be why I see so many things (rubygems, in particular) that try to call `brew --prefix` to fail.

I believe this little patch would be adequate? I don't believe it should break anything.