Fix for #912

[vesrah]

Fix

Class 'MockDrake' incorrectly implements interface 'DrakeWithModels'. Types of property 'on' are
            incompatible. Type '(event: string, callback: Function) => void' is not assignable to type '(events: string,
            callback: Function) => Drake'. Type 'void' is not assignable to type 'Drake'.

Relevant issues

#912
#913

Before submitting

If your pull request includes changes to the library code:

• Add tests (in modules/ng2-dragula/src/spec/...) for any code you have changed
• Make sure the test suite passes
• Make sure the demo app isn't broken