feat(ruby): update ruby to 2.7.0

[rjherrera]

This PR updates ruby to 2.7.0.

Detailes

• sidekiq-scheduler was updated to 3.0.1 as 3.0.0 wasn't compatible with ruby 2.7.0 as that gem depends on libraries that are no longer bundled by default on ruby (thwait, e2mmap).
• In the ci build for potassium, a specific bundler version is set in the bundle install command (2.0.2) because the new image runs bundler 2.1.2 which breaks builds with a path related issue (Bundler 2.1.0 does not expose executables from gems without "bundle exec", while 2.0.2 does rubygems/bundler#7494).
• A ruby 2.7 image was created for project's CircleCI builds #3.

closes #254