show version more prominently on website

Created by: jvanasco

It took me a while to figure out the current bootstrap version.

'Currently v3.1.0' is light gray text on the footer, against a white background.

It's not mentioned anywhere else on the front page.

On the download page, it's only visible as the link's target in the browser's status bar.

Meanwhile, the 2.x branch site has 'v 2.3.2' clearly above the fold.