Fix issue causing .navbar-brand element height to be shorter than .navbar height

Created by: PHLAK

With Bootstrab 3.1.0 the .navbar-brand element is shorter than the surounding .navbar element. You can even see this in the current 3.1.0 docs:

This issue (introduced with commit e38fd138) causes there to be a 30px space at the bottom of the .navbar-brand element that is unclickable.

This patch fixes that by utilizing the @navbar-height variable for the .navbar-brand height.