... config ...
'assetManager' => [
'bundles' => [
'yii\bootstrap\BootstrapAsset' => [
'sourcePath' => '@bower/bootstrap-sass/assets',
'css' => [],
],
'yii\bootstrap\BootstrapPluginAsset' => [
'sourcePath' => '@bower/bootstrap-sass/assets',
'js' => ['javascripts/bootstrap.min.js']
],
],
],