DOC: Update min rust version in crate docs.

Rustdoc is a good place for the minimum rust version, because that way
this minimum rust version information is versioned (per crate version)
on docs.rs

Author: bluss

src/lib.rs

@@ -61,7 +61,7 @@
 //!     needs matching memory layout to be efficient (with some exceptions).
 //!   + Efficient floating point matrix multiplication even for very large
 //!     matrices; can optionally use BLAS to improve it further.
-//! - **Requires Rust 1.32**
+//! - **Requires Rust 1.37 or later**
 //!
 //! ## Crate Feature Flags
 //!