Remove support for 1.8.7 and perhaps all EOL Rubies

[ms-ati]

It now appears that, in addition to being end-of-life for years, Ruby 1.8.7 is no longer runnable on Travis CI.

For comparison, Github Actions supports no Rubies older than 2.1.

Therefore, it's now time to remove testing, and then support, for Ruby 1.8. In Docile, this will result in actual code changes, as code paths and choices do exist solely to support 1.8 today.

How to use this ticket

I'd like to use this ticket to capture (a) plans and notes on what code can change to remove this support, and (b) links to every PR that pertains to removing this support. The final PRs should be for a new minor version.

What about Ruby 1.9, etc?

Do you use Docile on an end-of-life ("EOL") Ruby version, such as 1.9.3? If so, please respond to this ticket to let me know. If nobody is actively following this project and using EOL Rubies, I'd like to consider removing support for all of them. However, I would prefer to continue to support any Rubies that are actually in use by folks following this project, so please respond if you would be impacted!

Checklist

• [x] Remove 1.8.7 and REE from Travis CI (see #61)
• [ ] singleton_class - stop using the 1.8.7 fallback pattern
• [ ] FallbackContextProxy#instance_variables - stop supporting Strings on 1.8
• [x] docile.gemspec and on_what.rb - remove all pinned dependencies for 1.8
• [x] README and any other documentation
• [ ] HISTORY.md
• [ ] ??? Please suggest more items below!

[ms-ati]

UPDATE: As of Sun Feb 7 2021, there have been no comments on this issue requesting continued support for any EOL Rubies.

Context: as of today, all Ruby versions prior to 2.5 are end-of-life (EOL)

We should start to move forward with plans to remove support for some or all of them.