Drop support for Ruby 1.8, and update development dependencies

This PR drops support for Ruby 1.8 (And JRuby-18mode). Why?

- Ruby 1.8 is no longer supported by the core team.
- Dealing with encodings while support Ruby 1.8 and 1.9+ is a nightmare.
- Dealing with the development dependencies is getting very complicated because many of them have dropped 1.8 support in their latest versions.

If we merge this, we can release it as a new minor or major version.

@alloy @rngtng what do you think?