Media responsive size tweak.

Temporary solution for this responsive issue https://github.com/learningequality/fle-site/issues/25
I think the problem is the logo is too wide, so as the width shrink, nav items doesn't responsive much, then disappear at 990px.  I made the logo to be the smaller version at 990px, so at least the nav items would shows up on most tablets.

![screen shot 2013-12-29 at 12 44 35 am](https://f.cloud.github.com/assets/1319955/1819134/c54a317c-7065-11e3-9c62-bde39117ce1c.png)