update #features to use native rollout method

[dfguo]

• get rid of monkey_patching
• use native #features method from rollout
• get rid of #add_feature

[dholdren]

would be cool if this got merged and released. the current mechanism for hooking into rollout seems a little strange.

[jrallison]

Hey @dholdren, the current mechanism was the only option when this project was written (2013).

Unfortunately, I'm not able to maintain this any longer. If someone would like to pick it up, I'm happy to talk about how that would work.

Cheers,
John

[nfedyashev]

@dfguo thank you so much for the diff!