Remove attr_accessible for Rails 4 (which Spree 2.1 requires)

Fixes #32
Moving to acts_as_commentable 4.0.1 -- which now has passing tests and is rails 4 compatible

Fixes #32
Move to spree 2.1.1

Fixes #32
Depend on any Spree 2.1.0 version
Depend on Spree 2.2.0.beta