Update models/location.js

Change the function exported by `models/location.js` to conform to
the new contract expected by loopback-boot:
 - the function exported has a single argument only
 - the base class is accessed via `RentalLocation.base`

Requires strongloop/loopback-boot#26 to be released as loopback-boot@2.0.0-beta4.

/cc @raymondfeng @ritch