Binstubs generated with `bundle exec spring binstub --all`.
This allows to type `bin/rails`, `bin/rake` or `bin/rspec` and have it
using Spring (rather than `bin/spring rails`).
On my machine it produces a nice speedup:
- `rake -T`: 5.6s
- `bin/rake -T`: 0.25s
The startup speedup for `rails` and `rspec` is similar.