message: Add dates to RELEASES.md
diff: -# Release 0.2.0
  +# Release 0.2.0 (2018-05-25)
  ### Enhancements
  [48]: https://github.com/rust-num/num-bigint/pull/48
  [49]: https://github.com/rust-num/num-bigint/pull/49
  -# Release 0.1.44
  +# Release 0.1.44 (2018-05-14)
  - [Division with single-digit divisors is now much faster.][42]
  - The README now compares [`ramp`, `rug`, `rust-gmp`][20], and [`apint`][21].
  [21]: https://github.com/rust-num/num-bigint/pull/21
  [42]: https://github.com/rust-num/num-bigint/pull/42
  -# Release 0.1.43
  +# Release 0.1.43 (2018-02-08)
  - [The new `BigInt::modpow`][18] performs signed modular exponentiation, using
  the existing `BigUint::modpow` and rounding negatives similar to `mod_floor`.
  [18]: https://github.com/rust-num/num-bigint/pull/18
  -# Release 0.1.42
  +# Release 0.1.42 (2018-02-07)
  - [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].
  