spring is a dependency of spring-commands-cucumber.

Hi Jon, thanks for Spring! I hope it makes it into Rails 4.1.

Anyhow, I'd like to have my Gemfile read just
```ruby
gem "spring-commands-cucumber"
```
instead of
```ruby
gem "spring-commands-cucumber"
gem "spring"
```

Seems redundant to have to specify `spring` as well. What do you think?