Fix Sass minor rounding issues with width of grid columns

Allows Dart Sass to calculate column widths to a higher 10dp precision

one-third = 330px (not 329.98px)
two-thirds = 660px (not 659.98px)

src/govuk/settings/_measurements.scss

@@ -19,11 +19,11 @@ $govuk-page-width: 960px !default;
 /// @access public
 
 $govuk-grid-widths: (
-  one-quarter: 25%,
-  one-third: 33.3333%,
-  one-half: 50%,
-  two-thirds: 66.6666%,
-  three-quarters: 75%,
+  one-quarter: (1 / 4) * 100%,
+  one-third: (1 / 3) * 100%,
+  one-half: (1 / 2) * 100%,
+  two-thirds: (2 / 3) * 100%,
+  three-quarters: (3 / 4) * 100%,
   full: 100%
 ) !default;