Fix: mongoid_slug:set rake task for Mongoid 5. [DON'T MERGE]

[dblock]

Need to make sure the mongoid HEAD build passes, right now there're some issues with mongoid/observers, see mongoid/mongoid-observers#2.

[Fudoshiki]

#216 Tests against mri 2.3.1, rails 5.0.0.rc1

• mongoid tests passed Update MongoDB versions, target on Rails 5.0.0.rc1 mongodb/mongoid#4288
• mongoid_paranoia tests passed [Complete] Rails 5.0.0 support  simi/mongoid_paranoia#45
• mongoid-observers tests passed Mongoid HEAD Compatibility mongoid-observers#2
• mongoid-slug tests passed

[Fudoshiki]

@dblock want we target master branch at Mongoid HEAD?

[dblock]

Yes @Fudoshiki, but generally I am not too excited about trying to keep up with mongoid until there's some RC or something. Of course I'd take pull requests if someone is motivated to use the edge version(s).

[Fudoshiki]

@dblock
Can we drop Mongoid 4/5 and ruby < 2.2.2 support in master branch?

[dblock]

@Fudoshiki I'd like not to, unless we must because we just can't make it work.

[Fudoshiki]

ok

[Fudoshiki]

@dblock We can close this

[dblock]

Closing in favor of #219 .