Text.Pandoc.Data: store paths in dataFiles using posix separators. This way we have uniform separators, whether on Windows or Linux. This should solve a problem where on some Windows versions the data files weren't being found (#2459).
Note: This release does nothing more than fix a problem with some Windows builds of 1.15.1. Linux and OSX users may continue to use pandoc 1.15.1, so binary packages for these platforms are not provided here.