Convert <file src="" to native path for PublishedAssembly

Currently NugetSpecTester.should_read_the_name and NugetSpecTester.should_read_all_the_dependencies both fail due to incorrectly parsing of the path "..\..\src\FubuMVC.Core\bin\release\FubuMVC.Core.*", located in FubuMVCNuspecTemplate.txt, on 'unix'.

Following patch just ensure we convert the nuget spec file into 'native' file paths.

 As you can see the line a couple above my change 

var path = target.Replace('\\', '/');

is still hardcoding the slash.   I have left this as is because that would break tests and im not entirely sure I understand its "use case" yet and therefore would probably break it even more.