If you do not need to load bootstrap from your server, you can load the latest version from CDN (in most cases using CDN will provide extra loading speed).
Inside your config files ( web.php in basic and main.php in advanced ) you can tell your asset manager to load bootstrap from CDN instead from your server.
'components' => [
//other config
// if you decide to use bootstrap and jquery from CDN, uncomment assetManager settings
'assetManager' => [
'bundles' => [
// use bootstrap css from CDN
'yii\bootstrap\BootstrapAsset' => [
'sourcePath' => null, // do not use file from our server
'css' => [
'https://maxcdn.bootstrapcdn.com/bootstrap/3.3.0/css/bootstrap.min.css']
],
// use bootstrap js from CDN
'yii\bootstrap\BootstrapPluginAsset' => [
'sourcePath' => null, // do not use file from our server
'js' => [
'https://maxcdn.bootstrapcdn.com/bootstrap/3.3.0/js/bootstrap.min.js']
],
// use jquery from CDN
'yii\web\JqueryAsset' => [
'sourcePath' => null, // do not publish the bundle
'js' => [
'//ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js',
]
],
],
],
], // components