allow for active model serializer 0.9

When installing some gems that may be using Active Model Serializer version 0.9 (*cough* [Spree Wombat](https://github.com/spree/spree_wombat/blob/2-2-stable/spree_wombat.gemspec#L17)), Bundler is not able to resolve the dependencies.

To fix this, I simply add a minimum version (0.8.1) and a maximum version (0.10.0).