Fix comparison in registration model for year and programme

Ghost User
2018-10-03T20:15:44+02:00

~bug

Closes #693

#### Previous behaviour
Steps to reproduce:

1. Create a registration for a supporter
2. Do _not_ enter a study programme
3. Save
4. Error

#### New behaviour
Steps to validate that it works:

1. Create a registration for a supporter
2. Do _not_ enter a study programme
3. Save
4. Done!