Fix Sass rounding issues with width of grid columns

[colinrotherham]

This PR changes our grid column percentage sizes so they're rounded to the maximum precision available to Sass

By rounding ⅔ to 66.6666% 66.6667% (note the 67 not 66) we see browsers render our one-third and two-thirds grid column widths to whole pixels, which fixes #2361

It appears that no change in node-sass decimal precision is necessary 🎉

But we can go further by calculating two-thirds: (200% / 3) to add another decimal place:

Before

.govuk-grid-column-two-thirds {
  width: 66.6666%;
  float: left;
}

After

.govuk-grid-column-two-thirds {
  width: 66.66667%;
  float: left;
}

Note: Unlike LibSass (and node-sass) Dart Sass will output 66.6666666667% with 10 decimal places of precision

[colinrotherham]

@owenatgov Pushed up a CHANGELOG entry. Thanks

(Also noticed a previous fix was missing a PR number, let me know if that change is unwelcome and I'll remove)

[owenatgov]

Whoops. Thanks for fixing that 😁

[colinrotherham]

The Node.js v4 download for our LibSass v3.3.0 check is throwing an HTTP 500 😶

[colinrotherham]

The Node.js v4 download for our LibSass v3.3.0 check is throwing an HTTP 500 😶

It's back up now, checks all passing