Update flipper to 0.11.0 and take advantage of the new features

- Added an ActiveSupport (using Rails.cache) caching adapter
- Overview of the new features can be found at https://johnnunemaker.com/flippin-features-at-runtime/
- Full Changelog can be found at https://github.com/jnunemaker/flipper/blob/v0.11.0/Changelog.md



Signed-off-by: Rémy Coutable <remy@rymai.me>