RegistryUtil path keys should be updated to reflect TFM changes

[Pranav K]

From:
https://github.com/aspnet/BrowserLink/blob/master/src/Microsoft.VisualStudio.Web.BrowserLink.Loader/RegistryUtil.cs#L31-L33

#if DNXCORE50
        private static readonly string PathName = "dnxcore50";
#elif DNX451
        private static readonly string PathName = "dnx451";
#endif

The keys should probably change to reflect the new TFMs netstandard1.5 and net451.

[Joe Davis]

I'll make the registry key changes in WTE and then update these here,