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.

bin/rake

@@ -1,4 +1,9 @@
 #!/usr/bin/env ruby
+begin
+  load File.expand_path('../spring', __FILE__)
+rescue LoadError => e
+  raise unless e.message.include?('spring')
+end
 require_relative '../config/boot'
 require 'rake'
 Rake.application.run