CI: Use only in-support Ruby versions

[olleolleolle]

This cleanup PR prepares the CI matrix to get tests.

• CI: Remove every version which is listed EOF on the Ruby homepage: https://www.ruby-lang.org/en/downloads/branches/

• Allow Bundler 2 to be used in CI and in development.

  > Bundler could not find compatible versions for gem "bundler": > > In Gemfile: > bundler (~> 1.5) > > Current Bundler version: > bundler (2.2.15)

• Enable TravisCI bundler caching

• Add an empty Rakefile explaining that there are no tests.