[script] checkout the correct release

Currently in the install script, the tag dumux-course-2018 is checked out. In the handbook, @hommel and I@nedc both reference a tag called releases\3.0 for the dumux3.0 release. When the dumux3.0 is finished, this can be pushed and the correct release tag will be checked out. I referenced this in the handbook, and figured it would be good to have this ready in the dumux course install script before we forget. @timok releases\3.0 is what this will be called, correct?