Preselect the current issue tracker with selected="selected"

The previous behavior was, that the first element of the select was preselected, thus upon saving the project, the previous selected could be overwritten.

before:
![issue_tracker](https://f.cloud.github.com/assets/327488/1454178/73288864-4304-11e3-8424-45f88c65e9ed.gif)

after:
![issue_tracker_new](https://f.cloud.github.com/assets/327488/1454180/7a3d9130-4304-11e3-8fe9-be238747aaea.gif)