changes for arel 5 / AR 4.1

### Made changes necessary for running the gem with Rails version 4.1: 

Arel ```compile_update``` method signature changed from:
```
Arel::Crud#compile_update(values)
```
to:
```
Arel::Crud#compile_update(values, pk)
```

All specs are passing with ActiveRecord 4.1 now.