Merge pull request #3351 from mongoid/remove_im

Remove IdentityMap for good
Don't require rack/mongoid. It was removed with IdentityMap (7180173).

Fix #3367.
Require ostruct in lib/mongoid/indexable.rb to load OpenStruct.
Merge pull request #3368 from simi/fix-rack

Don't require rack/mongoid. It was removed with IdentityMap (7180173).
Merge pull request #3370 from simi/require-ostruct

Require ostruct in lib/mongoid/indexable.rb to load OpenStruct.
Reduce duplicated MRI Travis builds.
Merge pull request #3371 from simi/reduce-travis

Reduce duplicated MRI Travis builds.
Test ruby-head on Travis CI.
Merge pull request #3373 from simi/test-ruby-head

Test ruby-head on Travis CI.
Simplify Coveralls code
Remove pending test based on identity map.
Merge pull request #3374 from simi/remove-identitymap-pending-test

Remove pending test based on identity map.
Remove some pending tests for record_timestamps.

On mongoid we have Model.timeless, which can make simulate the same
behaviour as record_timestamps on AR, with a bonus of being thread-safe.
Remove multiple jdk tests on CI
Small cleanup on accessor
Small indentation [ci skip]
Add #one alias for #first method in findable.rb file
Add specs for Model#first and Model#one methods
Merge pull request #3385 from hbakhtiyor/alias-one-for-first

Add #one alias for #first method in findable.rb file
:scissors: