Added conversion to string (fixes bug with recent releases of Ruby)

Using recent versions of Ruby, you get the following error (tested on Ruby 1.9.2-p290):

```
NoMethodError: undefined method `gsub' for :@distance:Symbol
```

I suspect that with recent versions, ```instance_variables``` returns a Symbol therefore #gsub is not available.  This should work in all releases because if we get it as a string, it'll remain a string.

Thanks for the great gem.