rails3: fix route generation on course roster page

Change-Id: I7a1038360abf121b3c305f728c1969b5fc852bdb
Reviewed-on: https://gerrit.instructure.com/31288
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>