Find .bower.json if it exists

I'm using this module to find bower config files. With bower@1, it no longer works for installed dependencies because bower generates a .bower.json file, which this module doesn't know to look for.

After this PR, the following code in bower can use this module instead of hardcoding the `".bower.json"` string:

````js
            // Remove existent and copy canonical dir
            promise = Q.nfcall(rimraf, dst)
            .then(copy.copyDir.bind(copy, decEndpoint.canonicalDir, dst))
            .then(function () {
                var metaFile = path.join(dst, '.bower.json');
````