Upgrade valencia to use the new gradient syntax

[toddparker]

The gradient code is outdated and needs to be ported into the new style seen in the default theme. Looking for help with this so comment if anyone wants to take this on and submit a pull request.

[hotTea]

Hi Todd,
From the details in the jQuery Mobile blog, it seemed to me that you wanted something similar to this commit (e5e198a8dbccbb11995a) for ~/themes/valencia/jquery.mobile.theme.css. If so, please review my pull request. Thanks.