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'
```