Use @for instead of @while for WVU Slash positition modifier classes

[adamjohnson]

As mentioned in a comment in #273, the @while loop used in _wvu-slash.scss breaks gulp-dart-sass/gulp-sass with the Dart Sass compiler.

Current code:

Lines 76-78 and 100 of _wvu-slash.scss.

$i: 1;
@while $i < 5 {
  .wvu-slash { ... }
  $i: $i + 1;
}

I have a feeling this is due to either calc(-25% - (#{$i} * #{$spacer})) or ($i * $spacer).

The fix:

@for $i from 1 through 5 {
  .wvu-slash { ... }
}