Change bs require version

[N1cE-me]

Change pls in dev branch (1.0.x-dev) bootstrap require to 2.1.x-dev or 2.1.0 in Composer.
I tried add "prefer-stable": true but nothing...
I got same error:

 Problem 1
    - Can only install one of: yiisoft/yii2-bootstrap[2.0.x-dev, 2.1.x-dev].
    - Can only install one of: yiisoft/yii2-bootstrap[2.0.x-dev, 2.1.x-dev].
    - Can only install one of: yiisoft/yii2-bootstrap[2.0.x-dev, 2.1.x-dev].
    - 2amigos/yii2-file-upload-widget 1.0.x-dev requires yiisoft/yii2-bootstrap ~2.0.0 -> satisfiable by yiisoft/yii2-bootstrap[2.0.x-dev].
    - Installation request for 2amigos/yii2-file-upload-widget 1.0.x-dev -> satisfiable by 2amigos/yii2-file-upload-widget[1.0.x-dev].
    - Installation request for yiisoft/yii2-bootstrap 2.1.x-dev -> satisfiable by yiisoft/yii2-bootstrap[2.1.x-dev].

[tonydspaniard]

Can you help me with a PR? Not much time available lately