Handling file:// urls. This extends support to UNC paths.

[nithinphilips]

Patch to extend support to boxes stored on network paths (UNC paths).

We have some Vagrant boxes stored on windows network drives. When entering UNC paths into Vagrant they have to be in the file:// url format, but Ruby's File.mtime cannot handle these. This patch converts any file:// urls to proper UNC paths that can be handled by File.mtime.