yii2小部件不是通过composer安装的

Can't able to install widget using composer for yii2 framework

My composer look like this:

"require": {
    "php": ">=5.4.0",
    "yiisoft/yii2": "~2.0.6",
    "yiisoft/yii2-bootstrap": "~2.0.0",
    "yiisoft/yii2-swiftmailer": "~2.0.0 || ~2.1.0"

},
"require-dev": {
    "yiisoft/yii2-debug": "~2.0.0",
    "yiisoft/yii2-gii": "~2.0.0",
    "yiisoft/yii2-faker": "~2.0.0",
    "codeception/base": "^2.2.3",
    "codeception/verify": "~0.3.1"
},

You need to change your constraint in the composer.json to the following

"require": {
    "php": ">=5.4.0",
    "yiisoft/yii2": "~2.0.6",
    "yiisoft/yii2-bootstrap": "~2.0.0",
    "yiisoft/yii2-swiftmailer": "~2.0.0"
},
"require-dev": {
    "yiisoft/yii2-debug": "~2.0.0",
    "yiisoft/yii2-gii": "~2.0.0",
    "yiisoft/yii2-faker": "~2.0.0",
    "codeception/base": "^2.2.3",
    "codeception/verify": "~0.3.1"
},

As according to the DOCS

Note: Version 2.1 of this extensions uses Swiftmailer 6, which requires PHP 7. If you are using PHP 5, you have to use version 2.0 of this extension, which uses Swiftmailer 5, which is compatible with PHP 5.4 and higher.