message: Add rand 0.5 to the release notes
diff: - [`BigInt` now supports assignment operators][41] like `AddAssign`.
  - [`BigInt` and `BigUint` now support conversions with `i128` and `u128`][44],
  if sufficient compiler support is detected.
  +- [`BigInt` and `BigUint` now implement rand's `SampleUniform` trait][48].
  - The release also includes other miscellaneous improvements to performance.
  ### Breaking Changes
  - [`num-bigint` now requires rustc 1.15 or greater][23].
  - [The crate now has a `std` feature, and won't build without it][46]. This is
  in preparation for someday supporting `#![no_std]` with `alloc`.
  -- [The `rand` and `serde` dependencies have been updated to 0.4 and 1.0][24]
  - respectively, and neither are enabled by default. The `rustc-serialize` crate
  - is no longer supported by `num-bigint`.
  +- [The `serde` dependency has been updated to 1.0][24], still disabled by
  + default. The `rustc-serialize` crate is no longer supported by `num-bigint`.
  +- [The `rand` dependency has been updated to 0.5][48], now disabled by default.
  + This requires rustc 1.22 or greater for `rand`'s own requirement.
  - [`Shr for BigInt` now rounds down][8] rather than toward zero, matching the
  behavior of the primitive integers for negative values.
  - [`ParseBigIntError` is now an opaque type][37].
  [41]: https://github.com/rust-num/num-bigint/pull/41
  [44]: https://github.com/rust-num/num-bigint/pull/44
  [46]: https://github.com/rust-num/num-bigint/pull/46
  +[48]: https://github.com/rust-num/num-bigint/pull/48
  # Release 0.1.44
  