change api update_attributes -> update

AR 4 has changed their api, as mongoid wants to be as compatible as possible,
would be nice to update the update api as well.
[fixes #2966]
Updating changelog
document.atomically can take multiple of the same operation. fixes #3057
Update changelog
add `mongoid:drop_undefined_indexes` rake task

removes indexes that exist in the database but aren't specified on the
models
Update changelog
Refactor spec, rename for consistency
Prefix relation instance variables with _:

[ fix #3116 ]
Only flatten 1 level in push and add to set:

[ fix #3093 ]
small fix on ruby_version check

The ruby version check could happen before Rails.logger gets intialized
so we cannot assume that Rails.logger is not nil.
Example: https://travis-ci.org/errbit/errbit/jobs/8719703