message: fix version number in docs
diff: //!
  //! ```toml
  //! [dependencies]
  -//! argmin = "0.1.5"
  +//! argmin = "0.1.6"
  //! ```
  //!
  //! ## Optional features
  //!
  //! ```toml
  //! [dependencies]
  -//! argmin = { version = "0.1.5", features = ["ctrlc", "ndarrayl"] }
  +//! argmin = { version = "0.1.6", features = ["ctrlc", "ndarrayl"] }
  //! ```
  //!
  //! These may become default features in the future. Without these features compilation to
  