Drop reference to $margin variable in tap-highlight mixin.

The tap-highlight mixin references `$margin` which doesn’t exist without Vellum. Since it’s dubious whether blur size should be linked to margin, this PR drops it, replacing it with a static `5px` (what the var from Vellum would compile to). Since it’s the default value for a param, this should be fine.

Status: **Ready to merge**

Reviewers: @kpeatt 
Ticket: Closes #13 

## Changes
- Replaces use of `$margin / 2` with `5px` in the `$box-shadow-blur` param to the `tap-highlight` mixin.