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."
Merge pull request #275 from bbodenmiller/patch-4

don't need to keep repositories.config in DVCS