Address Backbone 1.1.0 compatibility.

This PR addresses Backbone v1.1 compatibility in #392. 

The removal of `_configure` means that we have to do some hacky constructor overriding to make LM work with v1.1.

Please take a look and let me know if you have any better ideas. I'm not happy with the elegance of it but it passes all tests on Backbone v1.1 and v1.0.