add Bootstrap 4 support

[hoaaah]

No description provided.

[leandrogehlen]

Thanks for PR.
I think this is not a good choice.
Maybe should better remove Bootstrap dependency and the developer can do this choice in your project

[hoaaah]

This PR is quick fix when you want to use BS4 instead of BS3 that comes with Yii2 by default. Of course this is an options to use BS4, if developer want to stick with BS3 they can easily ignore bsVersion params.

Without this PR, developer who wants to use BS4 can't use this extension because it keeps call BS3 Asset.