0.6.0

Added

• optional_rust_extension option - prints a warning to STDERR instead of raising an exception, if
  Cargo is unavailable and github_releases is either disabled or unavailable. Useful for projects
  where either fallback code exists, or a native extension is desirable but not required. (#4, #6)

Fixed

• cargo was not being run in the context of the rust project toplevel directory (#7, #8)