bower.config.json is obsolete

As pointed out by:
https://github.com/bower/bower/blob/e1d77e12b97a6ca025c18b1d7ddf617636c39ed6/lib/config.js#L5
bower.config.json is no longer supported.
looking around bower source code it seems that assuming 'bower.json' is safe enough.
I fixed the tests accordingly.