Remove .gradient mixin.
Update viewport-units-buggyfill from 0.4.2 to 0.5.0.
Merge pull request #1569 from vogdb/issue-1569

Update viewport-units-buggyfill from 0.4.2 to 0.5.0.
Merge pull request #1555 from vogdb/issue-1555

Remove .gradient mixin.