message: add a note about rand to 0.1.42
diff: - [num-bigint now has its own source repository][num-356] at [rust-num/num-bigint][home].
  - [`lcm` now avoids creating a large intermediate product][num-350].
  - [`gcd` now uses Stein's algorithm][15] with faster shifts instead of division.
  +- [`rand` support is now extended to 0.4][11] (while still allowing 0.3).
  -**Contributors**: @cuviper, @Emerentius, @mhogrefe
  +**Contributors**: @cuviper, @Emerentius, @ignatenkobrain, @mhogrefe
  [home]: https://github.com/rust-num/num-bigint
  [num-350]: https://github.com/rust-num/num/pull/350
  [num-356]: https://github.com/rust-num/num/pull/356
  +[11]: https://github.com/rust-num/num-bigint/pull/11
  [15]: https://github.com/rust-num/num-bigint/pull/15
  