Update README with "new timing"

After the changes introduced on https://github.com/progrium/dokku/pull/168 it no longer takes 5 minutes to bootstrap the machine.

Down here on a 35mb connection it took around 48 minutes to install the dependencies and build the base image, I'm guessing something around 30 minutes to 1 hour depending on users internet connection, maybe less but I'm pretty sure it won't be just 5 minutes :D