Fix file scanner.

'Label based on Current Word' Command failed with:

```
/Users/inz/Library/Application Support/TextMate/Managed/Bundles/LaTeX.tmbundle/Support/lib/LaTeXUtils.rb:204:in `recursive_scan': undefined method `each_with_index' for #<String:0x007ff981024d60> (NoMethodError)
	from /Users/inz/Library/Application Support/TextMate/Managed/Bundles/LaTeX.tmbundle/Support/lib/LaTeXUtils.rb:234:in `label_scan'
	from /Users/inz/Library/Application Support/TextMate/Managed/Bundles/LaTeX.tmbundle/Support/lib/LaTeXUtils.rb:57:in `get_labels'
	from Label Based on Current Word / Selection…:8:in `<main>'
```

This is on 10.9 Mavericks with system ruby `ruby 2.0.0p247 (2013-06-27 revision 41674) [universal.x86_64-darwin13]`.