Rails 6 Support

Currently, `lines-engine` doesn't support Rails 6. Is support for Rails 6 in the timeline?

```
Bundler could not find compatible versions for gem "rails":
  In snapshot (Gemfile.lock):
    rails (= 6.0.2.1)

  In Gemfile:
    lines-engine was resolved to 1.1.5.3, which depends on
      rails (>= 3, < 5)
```
