Versions 0.1.x are incorrectly listed as compatible with Ruby 2.3

[miloprice]

All versions of base64 use String#unpack1, introduced in Ruby 2.4.

In #17, the gemspec was updated to declare Ruby >= 2.4 as a dependency, but this change was only published in base64 v0.2.0. As a result, v0.1.0 and v0.1.1 are still published as being compatible with Ruby 2.3, despite causing base64 decoding to fail when run in a Ruby 2.3 app.

To remedy this, would it be possible to do either of the following:

1. Yank v0.1.0 and v0.1.1, since they have incorrect dependencies declared, and publish new versions v0.1.0.1 and v0.1.1.1 with correctly declared Ruby >= 2.4
2. Publish a Ruby 2.3-compatible version v0.1.2 that doesn't rely on String#unpack1

If the second option would be preferable, I can put up a PR.

[hsbt]

Done https://github.com/ruby/base64/releases/tag/v0.1.2