Fix overrides without main field

bower-files fails if the `bower.json` file specifies an `overrides` setting for a package that does not include a `main` property for that package:

     Error: Invalid glob argument: Error: No main property: "PACKAGE-NAME". Add to the overrides property in your component package file.

The attached pull request fixes the problem by replacing the tertiary assignment operator with a fall-through assignment that has the desired behavior (use `overrides.main` if defined, else use `component.main`)