Make used `NSBundle` overridable

[kimar]

When using ObjectiveRecord as a dynamic framework, NSBundle.bundleForClass(self) returns the framework's bundle which then, in turn, won't yield the correct CoreData model file. By overriding the bundle when using CoreDataManager this can be fixed.

[rohan-panchal]

This seems to have been abandoned. But i agree with this. I use ObjectiveRecord within a private cocoa pod and the fetching of the ManagedObjectModel needs to be done within a bundle within a framework within the main bundle.

The travis issue if i remember correctly has to do with Travis changing the ruby version on their OS X images.