Fix levels controller test (was failing on multiple test suite runs)

Fixes:

```bash
  1) Failure:
LevelsControllerTest#test_should_create_level [/Users/brian/code/cdo-curriculum/projects/dashboard/test/controllers/levels_controller_test.rb:28]:
--- expected
+++ actual
@@ -1 +1 @@
-"{ \"url\": \"http://test.host/s/5/level/1\"}"
+"{ \"url\": \"http://test.host/s/5/level/340\"}"
```