Fixed: config.ru evaluated twice

This PR fixes a problem with config.ru evaluated twice on startup. The cause of it was #rack object evaluated once in the scope of Ripl::Rack::Commands and then in global scope.

To reproduce:
```
# config.ru
puts "config.ru evaluated <= !!!"

run lambda { |env| [200, {}, ["OK"]] }
```
```
$ ripl rack
config.ru evaluated <= !!!
Loading development environment (Rack 1.2)
>> rack
config.ru evaluated <= !!!
...
```

This can lead to undesired behavior. In my project the whole middleware stack was loaded twice. Yuck!