Update release date.

Should not use `assert` with expressions with side-effect; ref mruby#…

…4981

`assert()` can be completely removed when `NDEBUG` is set.

Update version to `2.1.1`. (mruby 2.1.1 RC)

Release `mruby 2.1.0`.

Update version to `2.1.0`. (mruby 2.1.0 RC)

Update version and release date.

`mruby 2.0.1 (2019-4-4)`

Update release date.

Set the mruby-1.4.1 release date to `2018-4-27`.

Set the mruby-1.4.0 release date to `2018-1-16`.

Set the mruby-1.3.0 release date to `2017-7-4`.