Partial `before(:all)` compatibility

Adds `before(:all)` blocks compatibility for simple cases - when RSpec is configured as:

```ruby
 config.before(:each) do
    DatabaseRewinder.start
  end

  config.after(:each) do
    DatabaseRewinder.clean
  end
```
and when there is only one `before(:all)` block on the top. More complex cases, with multiple `before(:all)` and `before(:each)` blocks are not supported and I guess fall off the DatabaseRewinder simple philosophy.