Fix Webpack build with Bulma v0.6.0

[dhargitai]

No description provided.

[rolandschuetz]

There are two problems with the fix:

• The minimal required version is not defined in package.json (it should be 0.4.1, this is where the SASS change happened)
• The function sass is also required, this is where the $border is defined in newer versions

I created a working pull request:
#8

@everybody:
You can already use this fix with:

yarn upgrade vue-bulma-slider@https://github.com/rolandschuetz/slider.git