Fix loading of .rb locale files when `load_path` is not a string

[stevegeek]

Fixes #700

When load_path contains Pathname objects this is usually fine as things like File.open take them, however the one place is breaks is loading .rb locale files due to the 3rd param of Kernel#eval seemingly needing to be a String.

This fixes the issue by calling .to_s on the filename object before calling eval

[stevegeek]

Hey @radar , apologies for the poke, but just wondering if this PR needing anything else to get it merged? Thanks for maintaining the gem!

[radar]

Sorry to have missed this a few weeks back. Nothing else outstanding from my side here.

[stevegeek]

Thanks very much @radar !