Add MRI Ruby 2.1.2 to list of known rubies.

- Make new MRI Ruby 2.1.2 visible in the list of known rubies.
- Make MRI Ruby 2.1.2 the default when installing the Ruby 2.1 series.
- Demote MRI Ruby 2.1.1 to a known legacy version.
Remove unused patch levels from semantic versions.

- Starting with Ruby 2.1.0, Ruby now uses semantic versioning instead of
  patch levels.
- https://www.ruby-lang.org/en/news/2013/12/21/semantic-versioning-after-2-1-0/
  has details.
Add MRI Ruby 2.1.2 to list of known strings.