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.