Deprecated SCSS warnings from vendor code

[hughbris]

I was going to submit a PR until I realised the deprecation warnings I am seeing originate with included vendor code, which you may want to update by another process than random PRs.

So I'm seeing lots of

@elseif is deprecated and will not be supported in future Sass versions.
Use "@else if" instead.
311 │     @elseif $value == "space-around" {

and similar messages in scss/vendor/bourbon/css3/_flex-box.scss. They should be substituted or the library updated.