Update dependency version for active_model_serializers

Using Alpha1 breaks when adding a `has_one` relation and the relation is not set, throwing an error of `TypeError - no implicit conversion of nil into Array`
Modify first argument for Grape::Endpoint.new to fix error caused by calling a method on nil.
Merge pull request #25 from DanyHunter/patch-1

Update dependency version for active_model_serializers