Fix incompatibilities with DataMapper and Mongoid

Both DataMapper and Mongoid were attempting to include an undefined
module, Recommendable::Ratable::InstanceMethods. I had forgotten to add
this after seeing warnings from ActiveSupport::Concern that
InstanceMethods modules would no longer be included automatically.
Mongoid and DataMapper need explicit includes, however, so those modules
will be defined inline for them.

Fixes #58

Signed-off-by: David Celis <me@davidcel.is>
Bump version to 2.0.2

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