Add April 2013 Derby copy.

Unable to spot-check myself due to a Vagrant error, but I rarely have problems with changes like these.

To spot check:

* In Constance config, change *DEMOS_DEVDERBY_CURRENT_CHALLENGE_TAG* to *challenge:2013:april*
* Check that the copy shows up in the "Current" tab of the Dev Derby without issue (links work, etc.)
* In Constance config, change *DEMOS_DEVDERBY_CURRENT_CHALLENGE_TAG* to *challenge:2013:june*
* Nothing should show up in the "Current" tab, but some short copy should show up when you hover over "WebGL II".