always load Rack up front at startup

always load Rack up front at startup

It's too complicated and error-prone to allow apps to use a
different version of Rack than the one Unicorn would otherwise
use by default.

If an app requires a different version of Rack than what Unicorn
would load by default, it is recommended they only install that
version of Rack (and no others) since Unicorn does not have any
strict requirements on currently released Rack versions.

If it is not possible to only have one Rack version installed
globally, then they should either use Isolate or Bundler and
install a private version of Unicorn along with their preferred
version of Rack.  Users who install in this way are recommended
to execute the isolated/bundled version of Unicorn, instead of
what would normally be in $PATH.

Feedback/tips to mailto:mongrel-unicorn@rubyforge.org from
Isolate and Bundler users would be greatly appreciated.