Add rubocop binstub for spring support

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
```

bin/rubocop

@@ -0,0 +1,4 @@
+#!/usr/bin/env ruby
+load File.expand_path("spring", __dir__)
+require 'bundler/setup'
+load Gem.bin_path('rubocop', 'rubocop')

bin/spring

@@ -1,6 +1,6 @@
 #!/usr/bin/env ruby
 
-# This file loads Spring without using loading other gems in the Gemfile, in order to be fast.
+# This file loads Spring without loading other gems in the Gemfile in order to be fast.
 # It gets overwritten when you run the `spring binstub` command.
 
 if !defined?(Spring) && [nil, "development", "test"].include?(ENV["RAILS_ENV"])