Show 404 if course date format is wrong.

Resolves #169
Add GDS test mocks
Add test for course instance date format
Merge branch 'refs/heads/master' into fix-course-instance-error
Merge branch 'refs/heads/master' into fix-course-instance-error
Don't require mocha until later
Try putting the right gems in the right group.

Idiot.
Merge pull request #179 from theodi/fix-course-instance-error

Fix course instance error