I observed a significant speed-up on the second run. The first run seems
to be a lot slower though. Additional output suggests that it's now
loading the Rails environment which it doesn't do without spring.
```
time bundle exec rubocop Gemfile
0m2.496s
time rubocop Gemfile # not always right version
0m1.999s
time ./bin/rubocop Gemfile
0m7.543s
time ./bin/rubocop Gemfile
0m0.670s
```
