When loading active_experiments, it should not look into user's 'finished' keys

[andrehjr]

Fixes #580

All calls for experiments keys on User should be gated with keys_without_finished. This method filters out all 'finished' keys.

The issue happened because active_experiments was loading all user.keys. And when calling key_without_version method with a 'finished' key with more than 9 versions. Example:

irb(main):012:0> key_without_version('exp:11:finished')
=> "exp"
irb(main):013:0> key_without_version('exp:1:finished')
=> "exp:1:finished"

It would end up overriding the value of an experiment with the 'exp:11:finished' value from the Storage, which is always 'true'.