BUG: Raise ImportError if optional dependency speclite is not installed

[rrjbca]

Description

Attempting to call mag_ab without the optional dependency speclite installed will now correctly raise an ImportError instead of a NameError. Fixes #436

Checklist

• [x] Follow the Contributor Guidelines
• [x] Write unit tests
• [ ] Write documentation strings
• [ ] Assign someone from your working team to review this pull request
• [x] Assign someone from the infrastructure team to review this pull request