fix escaped ')' in sigil

This fixes #2840 

Now we have:

```
iex(1)> ~S(Interpolation?: #{3+0.14} ' '\))
"Interpolation?: \#{3+0.14} ' ')"
iex(2)> ~S(foo\))
"foo)"
```
@josevalim Thanks, good catch. I have update the patch to deal with other sigils.