fixes View.isInDOM access in RenderCache

After switching between several actions using redirects in the (now old) v0.15 release the application dies with an "undefined method" exception. The problem happens in `Batman.RenderCache._evictExpiredKeys()` accessing `Batman.View.isInDOM` as a method instead of a property.

While all unit tests happen to implement `isInDOM()` for the `MockView` the real `Batman.View` implementation is lacking one. However Batman.RenderCache tries to access it instead of the property `Batman.View.isInDOM`.

Being "default inactive" after the v0.16 release this should no longer be an application-breaking issue if one does not activate the RenderCache. 

Applying the patch should fix this issue.