use `4-0-stable` arel branch to get bundle working again.

this resolves the following error when running `bundle install`

```
ArgumentError: wrong number of arguments (2 for 1)
    /Users/senny/Projects/rails/.bundle/bundler/gems/arel-454a25f18c95/lib/arel/crud.rb:5:in `compile_update'
```

The build is still broken with lots of:

```
ArgumentError: wrong number of arguments (2 for 1)
    /Users/senny/Projects/rails/.bundle/bundler/gems/arel-454a25f18c95/lib/arel/crud.rb:5:in `compile_update'
```

Gemfile

@@ -12,7 +12,7 @@ gem 'bcrypt-ruby', '~> 3.1.2'
 gem 'jquery-rails', '~> 2.2.0'
 gem 'turbolinks'
 gem 'coffee-rails', '~> 4.0.0'
-gem 'arel', github: 'rails/arel', branch: 'master'
+gem 'arel', github: 'rails/arel', branch: '4-0-stable'
 
 # This needs to be with require false to avoid
 # it being automatically loaded by sprockets