Finalizer of ShellScript backend should not capture itself.

With the following code:
```ruby
        ObjectSpace.define_finalizer(self) {
          File.write("spec.sh", @lines.join("\n"))
        }
```
the proc captures `self` and `self` is never finalized.
(Actually, this program works properly because `self` is finalized when the program finishes)

See Also (Japanese): http://docs.ruby-lang.org/ja/2.0.0/method/ObjectSpace/m/define_finalizer.html