message: use dyn trait to fix nightly build
diff: @@ -45,7 +45,7 @@ impl Error for IoError {
  }
  }
  - fn cause(&self) -> Option<&Error> {
  + fn cause(&self) -> Option<&dyn Error> {
  match *self {
  IoError::Io(ref err) => Some(err),
  IoError::BadMatrixMarketFile => None,
  