title: How to update single rails engine (gem) within a rails app?
body: <p>There are multi rails engines (gem) in our rails 3.2.12 app. When there is an update to a rails engine, the version is updated as well. Then <code>bundle update engine_name</code> to update the engine. However when <code>bundle update engine_name</code>, there is an error about <code>ActiveModel</code> which is not supposed to be updated. What's the right way to update a single rails engine gem?</p>