Fix simple interval calculation for descending intervals

Hi, I found a bug in the simple interval calculation for descending compound intervals (of a multiple of 7).

Here is an example:
```
teoria.interval("M-13").simple()
=> "M-6" // Expected.
teoria.interval("M-14").simple()
=> "M7" // Wrong! Should be "M-7".
```

Descending intervals of a multiple of 7 lose its negative sign. Attached is a fix and a quick test case for this scenario. Please let me know if my changes are appropriate.

Thanks :)
Sean