spring: add Spring to binstubs of default commands

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.

.rubocop.yml

@@ -4,6 +4,7 @@ require:
 AllCops:
   Exclude:
     - "db/schema.rb"
+    - "bin/*"
 
 Bundler/DuplicatedGem:
   Enabled: true