Make header image on labs site responsive

Should look a little nicer on smaller screens now. :)
Thanks for such an awesome framework, btw!
<img src="https://f.cloud.github.com/assets/401283/2179319/37c786de-96a2-11e3-9fe1-03b846063e1d.png" width="300" /> <img src="https://f.cloud.github.com/assets/401283/2179318/363d2652-96a2-11e3-8dd4-c1da4465103b.png" width="300" />