Don't load both Kaminari and will_paginate

It's possible users will have both `will_paginate` and `kaminari` as
dependencies. Because the two are incompatible, avoid loading both and
instead first attempt to load Kaminari. If that cannot load, then
attempt to load will_paginate. Fixes #18.

Signed-off-by: David Celis <me@davidcel.is>