v4: Change the default value of $font-size-sm to .875rem

Created by: tagliala

Hi,

I was taking a look at the new typography and I found that the only non-round value was the one generated from $font-size-sm.

$font-size-root:             16px !default;

$font-size-base:             1rem !default;    // 16px
$font-size-lg:               1.25rem !default; // 20px
$font-size-sm:               .85rem !default;  // 13.6px <--
$font-size-xs:               .75rem !default;  // 12px

$font-size-h1:               2.5rem !default;  // 40px
$font-size-h2:               2rem !default;    // 32px
$font-size-h3:               1.75rem !default; // 28px
$font-size-h4:               1.5rem !default;  // 24px
$font-size-h5:               1.25rem !default; // 20px
$font-size-h6:               1rem !default;    // 16px

$display1-size:               3.5rem !default; // 56px
$display2-size:               4.5rem !default; // 72px
$display3-size:               5.5rem !default; // 88px
$display4-size:               6rem !default;   // 96px

This PR changes the default value of $font-size-sm from .85rem to .875rem, which gives us a round 14px font size.

Sorry if the 13.6px value was intended