Unintended behavior for determine()

This method works best when trying to guess certain values, but breaks on edge cases.

```
>>> intervals.determine("C", "Eb")
'minor third'
>>> intervals.determine("C", "Ebb")
'diminished third'
>>> intervals.determine("C", "Ebbb")
'diminished third'
>>> intervals.determine("C", "Ebbbbbb")
'augmented third'
>>> intervals.determine("C", "Ebbbbbbbbb")
'augmented third'
```