navBarHeightMobile hotfix

## Issue

```css
$topBarHeight: 75px
$topBarHeightMobile: 45px
```

![navBarHeightMobile-hotfix](https://f.cloud.github.com/assets/222064/105582/1eb1cebe-69b9-11e2-9e91-d59d2991e313.jpg)

![navBarHeightMobile-hotfix-2](https://f.cloud.github.com/assets/222064/105585/6680f4cc-69b9-11e2-8002-4e5d0f037843.jpg)

When the css variable navBarHeightMobile is set, it does not work correctly. Height of topbar and items is never changed when responsive.

## Solution

The following code will fix the issue, and make it work as expected.

**Please test this code first.** I had issues testing my hotfix branch. I believe my system is using a cached version or something.

Look at the code in this pull request and you will see what I am doing.

What lead me to editing this gem was this monkeypatch that resolved the issue locally.

```css
@media only screen and (max-width: $topBarBreakPoint)
  .top-bar
    min-height: $topBarHeightMobile
    & ul > li a:not(.button)
      height: $topBarHeightMobile
      line-height: $topBarHeightMobile
```

Signed-off-by: Keil Miller Jr <keilmillerjr@me.com>