Ruby library improvements

[mariusandra]

• Add feature flags support
• Is there any way to integrate with Rails to enable some basic autocapture?

[taobojlen]

For a Rails integration, it might be possible to draw inspiration from ahoy which demonstrates one way to auto-capture pageviews.

[rafaeelaudibert]

Rails was integrated on v3.5.0. See docs in posthog.com/docs/libraries/ruby-on-rails.