Improve radial gradients

Currently, the status circles for failed builds are invisible in browsers such as Presto-Opera and IE10, even though both of these browsers do support radial gradients.

This pull request adds flat fallback colors for older browsers and updates the unprefixed radial-gradient syntax to match the w3c spec as well as the actual implementation in Firefox, Chrome, IE10 and Opera.

I did not check in built versions of the CSS in order to prevent conflicts when merging.