using @line-height-computed in .navbar-nav link

When a user changed the @line-height-base/@line-height-computed var, the nav-bar items would not scale appropriately.

Screenshots (using custom font, which triggered the issue)

Before:

![before](https://f.cloud.github.com/assets/89551/906128/38e89574-fc71-11e2-8e8b-3e5c76a97f3f.png)

After:

![after](https://f.cloud.github.com/assets/89551/906129/3cc669d2-fc71-11e2-8a6e-8398e14e2c73.png)