add --with-weak-etag option

This fixes http://forum.nginx.org/read.php?2,240120,240120 / https://code.google.com/p/phusion-passenger/issues/detail?id=903 / http://trac.nginx.org/nginx/ticket/377 by passing through weak etags.
Weak etag can be generated in rails/rack apps using http://stackoverflow.com/questions/18693718/weak-etags-in-rails/21003624#21003624

The nginx team seems to be unwilling to accept our patch or do anything about this issue since a few months now. This may make it into nginx eventually, but we think having etags is a pretty vital feature for a webserver so we patched it in.

Can be activated using:
```
Package <| title == "boxen/brews/nginx" |> {
    install_options => ["--with-passenger", "--with-weak-etag"],
    require => File["/opt/boxen/homebrew/Library/ENV/4.3/passenger-config"]
  }
```

@fromonesrc @rafaelfranca