don't need to keep repositories.config in DVCS

As mentioned in https://github.com/NuGet/NuGetDocs/pull/262#issuecomment-56222507 "it's actually okay to omit the `repositories.config` from source control, as NuGet will re-generate it when you open the solution again." This however conflicts with information mentioned at https://stackoverflow.com/questions/7286261/nuget-repositories-config, https://stackoverflow.com/questions/7015149/multiperson-team-using-nuget-and-source-control, & https://github.com/github/gitignore/blob/master/VisualStudio.gitignore#L140. I believe those are all based on a dependency on `repositories.config` that existed previously but that new versions of NuGet & VS better generate it or work around not having it when needed.

Fixes https://github.com/NuGet/NuGetDocs/issues/175 & https://github.com/NuGet/NuGetDocs/pull/92#issuecomment-48626010.