Fix a bug that paginatable arrays with total_count option always returns whole array

Fixes #516
:sob:
Require mongoid/version before use

For some reason it is now required, but not sure why.
Use just '2.1' so travis picks up the newest ruby automatically

Closes #520
Use AS ~> 3.2.18 if Ruby is so old
Mongoid 3.0 doesn't play nicely  with AS 4.0 and later
Temp fix for this bug https://github.com/travis-ci/travis-ci/issues/2230
allow to pass :views_prefix to #paginate to make kaminari work in cells.
add K:Helpers::CellsHelper.
add a test for :views_prefix.