compatibility fix with fpm path-mapping feature

FPM::Package::Dir expects paths to behave as strings.
The [path-mapping][] feature depends on regex comparisons with
path names.

[path-mapping]: https://github.com/jordansissel/fpm/commit/8360506c394b18e568d17b66e650639b46fbae7b
Merge pull request #38 from skiold/fpm-path-mapping-fix

compatibility fix with fpm path-mapping feature