fixes tc39#49 - change view port marks

Author: zoepage

_sass/_variables.scss

@@ -10,5 +10,5 @@ $font-weight-regular: 400;
 $font-weight-semi-bold: 550;
 $grid-gutter: 40px;
 $spacing-paragraph: 40px;
-$bp-tablet: 660px;
-$bp-desktop: 920px;
+$bp-tablet: 920px;
+$bp-desktop: 1200px;